on XP:
I saw a developers post that mentioned this a bit. you get some for being in a restricted zone, but not much. you get more for being in cover in a restricted zone, and you get even more for MOVING between cover in a restricted zone.
I think you also get some bonuses based on alert level, and having your guys in a zone when you take an objective seems to give them a boost as well.
I find that playing the game normally lets me level up at a satisfying pace, and I dont think I'll max everyone before I hit the final zone, but I do have most of the advances that I WANT, and I think I'll get the rest of them by the time I finish all the other side missions.
So, seems properly paced.
On Reinforcements:
Different areas behave differently, but they seem to be consistent within that area. So observing for a little while can give you an idea what should happen.
I've noticed that the regular patrol for any given area seems to be capped at a certain number, so if you assassinate guys more will continue out.
But, if you brainjack them, and don't cause any other alarms or shoot out any lights, then you can basically make it a safe area.
My general method for hitting targets with minimal risk is to infiltrate as far as possible, brainjack some guards, have them shoot out whatever security systems look to be a problem while drawing massive crowds of reinforcements, and then let them die. Eventually when things die down, brainjack the regular patrol in the area until they are all owned, and walk in.