They do sound like they could be helpful. That gold sounds awesome

, however I am curious how much maintenance averages in that situation? I only know what I've read, and IIRC in the "the power of sushi" article (or an article it links to) it says that it can be difficult to turn a profit on the domestic branches of a corporation due to maintenance, and one is much more likely to turn a profit if one spreads one or more (relatively weak) corporations to an AI or several. Also there was a post somewhat recently to the effect that the scaling of corporation yields to game speed and (possibly?) map size means that you may not always get a lot of benefit from them.
Http://forums.civfanatics.com/showpost.php?p=8075413&postcount=89
The main power from (domestic) corporations is from the great local benefits in the cities that they are spread to. Your cities tend to become better when they have +10

and +15

.

Not that I'm saying that this will always be the case, but it can be if you get lots of corporation resources. The gold upkeep of the corporations is however fairly low compared to the gains that you can get through them. And if you do have the headquarters founded in a Wall Street city, then the costs are extremely low compared to the benefits.
@Roland: thank you for the suggestion. I'm not at the same level as you right now. I've been playing on Noble without Better AI or aggressive AI but it has started to seem pretty easy. My current game is a shuffle map, epic speed, and I took a random civilization and wound up with America (Lincoln). Granted that Lincoln seems to make a good warmonger with PHI for bulbing and CHA for promoting units... but still, I've been able to capture pretty much whatever I wanted whenever I wanted to. I'm getting to having about half of the industrial era techs, and only two AIs have had even one Renaissance tech. That tech was astronomy, and they haven't gotten very much benefit out of it (and Pacal, one of the two AIs, is already gone). They probably would've done better to get gunpowder, but they seemed to think that Astro and Divine Right were more important

.
My suggestion had less to do with the difficulty level. It just seems that combat is giving you a big edge in your games and tends to end your games early. If you want somewhat more balance and a longer game then these suggestions would help.
Bigger map: It typically takes about the same time to conquer a city, but when you capture 2 cities out of 20 of an AI on a huge map, then that will barely put a dent in that AI while if you capture 2 cities out of 6 of an AI on a small map, then that AI will be crippled.
Aggressive AI: The AI will focus more on military and less on economy. This can hurt the AI in the economic department a bit when you play on the lower levels. The AI barely notices the higher maintenance on the highest levels. The larger military will make it more costly to walk over AI's militarily and will make them capable to harm you militarily. So war becomes a relatively less attractive path to victory. It can even be so that it's best to let the other AI's fight while you try to stay out of it. However, I tend to do some aggressive fighting myself anyway. Wanna have some fun, right!
BetterAI: The mod improves the AI and thus makes the game harder. Especially the latest version seems to be a great leap in AI improvement. The gains in AI ability are militarily and economically. The military gains are more interesting to you, I guess.
Difficulty level: Of course, the difficulty level will make war tougher, but it more generally makes the game tougher, so it's not the change I was trying to suggest for your games.
I'm thinking I'll go for Prince difficulty together with BetterAI next. Is there still a little work involved (as I had read) in getting both BetterAI and BUG to work together, or have they been combined?
I know the mod creators of both mods are planning to release a merged version of both mods. They realised that the community tends to want them both at the same time but that many are incapable to do the merge themselves.
It's not so hard to merge BUG with BetterAI (there's a thread about it in the BetterAI forum). It is quite hard to merge BULL with BetterAI as both mods alter the dll file. Without quite some experience, I would advice you to stay away from that.
By the way, you should of course play the game how you like it. Maybe you like early conquest victories the most and then you should definitely not make any changes to the settings. I just thought I caught a hint of the feeling 'it's a shame I never experience the late game because I tend to win early'.