Yes, you're right, splitting does give a slight advantage (maybe also from faster xp gain due to lower GV?), though this is balanced out by the higher maintenance cost as you mentioned
That said, I'm not sure how it works with civics that give you X free units.
Actually, XP gain is faster, in effect, with larger units.
Although they gain less XP than smaller units, they gain more than 1/3rd the XP of a unit 1/3rd the size. Meaning the best strategy would actually be to merge a worker unit up to level up for a while, during which time it works on longer tasks (to minimize wasted effort), and then split it up for efficiency and less wasted effort...
Now, I've mentioned this before, but I saw it come up in the SVN Discussion again as it related to unit limits. Splitting units is currently a little bit OP'd- especially early on in the game, when it provides huge advantages for exploration.
I *STRONGLY* recommend increasing the strength-gain factor when merging units, and increasing the strength-loss when merging units (obviously by the same factor), in order to encourage the AI to merge units more often and stay away from the unit limit... In this SVN (for the coming release) if there's still time, in the SVN's for the next release if there's not.
There are also tactics that only larger groups can effectively utilize, like combat reserves (essentially, some of the men in a unit stay back from the fighting in order to preserve their energy, then advance and swap out with the other men in the unit who were actively fighting when they become tired. This improves the combat effectiveness of a unit fighting in a prolonged engagement vs. a "throw everything at them" approach, and is especially effective when fighting a superior-quality unit in real life: as it prevents the men on the front line from becoming discouraged) and tactical flanking (this being distinct from the kind of large-scale strategic flanking seen on the map, and especially effective when fighting an enemy of equal combat quality but inferior numbers- your own center will hold long enough for your flanking elements to come around the sides of the enemy force, but the enemy center won't break so soon as for the flanking forces to not have time to get into position).
Regards,
Northstar