I suspect that the use of one sphere of magic does throw off one's internal balance (probably just reinforcing an existing imbalance that led the mage to specialize and excel in said sphere to begin with), but any change extreme enough to cause a change like hair made of fire would certainly kill the mage first. (I don't mean such changes couldn't happen, but that that point the mage would have died and been processed into a demon.)
The physical elements of the spheres are but a representation of their deeper psychological aspects. Ice isn't actually about low temperatures, it is about stasis, stubbornness, and a passionless reactionary spirit. Fire isn't heat, it is passion. Dimensional isn't so much about phasing from one sphere to another as it is about a focus on defining things and though their relations to others instead of intrinsic worth. In general, you could expect focus on one sphere of magic to bring the caster's personality closer to that of the god of said sphere.
Air mana would make the user irresponsible. This wouldn't be malicious, but just a severe case of ADHD. It becomes hard to concentrate on what you are supposed to do, since there are a lot more fun and crazy things on which to waste your time.
Body magic would make the user have trouble controlling his base emotions. He will become increasingly lustful and irritable, and this growing misanthropy would make him a bully.
Creation mana would make the user more creative. It would increase intelligence, but probably also make the user want to create for the sake of creating, ignoring more practical needs.
Chaos mana would be rather similar to Body, but the misanthropy is without purpose. You don't dislike people because they get in the way of a strong desire, you just randomly decide to cause innocent people problems. Your behavior would become quite erratic.
Death mana would probably cause a dour attitude like Arawn's. I guess it could also lead to becoming gaunt, hair falling out, or skin peeling off, but this wouldn't be because of the mana so much as because a mater of death mana wouldn't like to go out very much and wouldn't take care of his body very well. Of course, it could also lead one to become a lich.
Dimensional makes one focus on relationships at the expense of any intrinsic value. Your worth becomes defined purely in terms of your power over others. The well being of yourself, your family, your kingdom, etc., doesn't matter, as long as as it is better off than that of everyone else. Loved ones become obsessions, and you don't like spending time with them you just need them to be there. All your relationships become highly codependent and unhealthy. You become extremely clingy and domineering, so others become less comfortable around you and want nothing to do with you. You have a very "you're either for me or against me" point of view, so this makes your loved ones enemies who must be destroyed.
Earth mana is the opposite of Air. It would make a user more responsible, and a harder worker. Users might become a bit dull, but would be very reliable and trustworthy. They might be a little stubborn, but that is mostly because they know their normal habits work.
Enchantment mana might make the users more charming and friendly
Entropy mana causes despair. It creates dreams of greatness that you don't really believe you can achieve, and drives you mad with desire. Users become self destructive towards these ends, and eventually just suicidal. They may have rashes etc., but this is mostly because they just aren't taking care of themselves. You can expect them to start cutting themselves, even when it isn't part of a spell. They are dangerous, as misery loves company.
Fire mana ignites all passions. Whatever a fire uses does, he will take it to an extreme. Users will become obsessive and reckless. Whatever a fire user believes is is more important than life, so they too may seem suicidal, but only where their death furthers a specific goal.
Force mana makes one care a lot about balance, so users would use it only in moderation and never really be changed a lot by it. They would become fairly sociable, and wouldn't really try to force their will on others but would be very upset when others break their agreements.
Ice mana makes users very very stubborn. They become passionless, but will defend their habits as strongly as if filled with great passion. Things will always seem to be changing too fast, and they will long to return to how things were when they were younger. They become inactive and bitter. Even while still young, they are essentially grumpy old men. Worrying about how things keep changing could cause premature aging, including white hair.
Law mana would make one strict, emotionless, and unyielding, to friend and foe alike. It can cause OCD, as following one's personal rituals becomes more important.
I'm not really sure about life. If I had to guess, I'd say it is rather similar to spirit, making one believe in second chances and being very willing to give up one's power. (Twice a god of life has chosen to give away his precept.) This may mean this one sphere is the opposite of addictive. If it makes the users not keep trying to use it, it may be rather hard to have it cause deeper psychological effects.
Metamagic probably makes users much more intelligent, but also very aloof from things around them. These scholars tend to miss out on actually living what they study.
Mind would make one more cunning, but mostly is leads to avarice and insanity. Users seek to own and control everything and everyone around. At first it greatly helps them achieve these goals, but it is unsustainable and when things mess up it drives the user mad.
Nature leads users to slowly but steady grow in to better, healthier, more mature people. It is continual and unrushed growth, so there isn't really a final state as one becomes wiser. Nature users would be quite patient with tohers as long as they see some progress.
Shadow mane use would make the user untrustworthy and suspicious of the evil intend in everyone else. Users will cheat more and more because they believe that everyone else is doing the same and they will fall behind if they don't. As cheating is part of the game those few who don't are just stupid and deserve to be exploited.
It seems like Spirit should make one a kinder, gentler, more caring person, but we have one very good example where that is not the case: Laroth. Laroth frankly seems about as unlike Sirona as one can get. I guess this goes to show that there is more to good and evil than mere spheres, and that mana can have the opposite effects based on the intention with which one uses it.
Sun mana should make one very curious and very open to all inquiry. Users will not be content with how things appear, but will want to illuminate things further so that everyone understands everyone else. This means users will become increasingly philosophical and more interested in discourse with friend and foe alike rather than attending to any practical concerns.
Water mana use is calming, soothing away all passion and making the users friendly to all they meet, but not particularly sociable. They become quite content with the status quo, rather than longing for change or for the good old days like fire or ice.
I suspect that focusing on opposing spheres isn't very common or easy to do because casters tend to focus on and are inherently best at spheres that are already over represented in their souls. I'd guess that if one really could master opposing spheres that each sphere would balance out the other's effect on the casters soul, thus keeping the user quite sane, healthy, and in general a very normal person. That is, of course, if it doesn't rip the user in two and lead to a severe case of multiple personality disorder, where each personality can only use one of the 2 dominant spheres.
