if i solo through the entire game chances are that i will hit the EXP cap right? but if i have 6 people in my party the whole time i probably won't. so if i set the difficulty to the highest setting does it increase the EXP i get from killing stuff? if so, could i have like 3-4 people in my party that at the hardest setting it maxes them out at the end? thereby making me super powerful(4 maxed out characters would be sweet)

IIRC, increasing the difficulty has no effect on the xp gained. Only if you decrease the difficulty, you will get less xp.

If you increase difficulty, you'll get more monsters, which could raise your experience. However, I've had no problems in getting my six characters to max out their experience in either SoA, or in the new experience cap in ToB. And if you feel you need still more opportunities in SOA alone, consider loading on TDD.

Just play the WHOLE game, doing all, or most of, the quests and your characters will max out on xp. Dont worry about that! And lowering difficulty will lower xp. Higher difficulties will not effect you xp from a single kill.