I've edited my game to take up to 7 in a party. So far on 4 different play throughs there has been little game stopping bugs overall. In fact it's hard to tell if it's my modification of the game that causes the bug or something that is a overall problem that will be addressed in official release. In most cases any game crashing seems to stem from a cutscene. In terms of game difficulty I have had less total party kills than with the standard 4 sized party, but still had complete party wipes in the bigger battles. In reality with the current available roster of party members fights are somewhat easier with a larger party. In most fights at least 1, if not more party members are knocked unconscious. Usually it is Astarion as I use him as an archer and everyone hates archers.
I have not seen a single instance where party/NPC dialog doesn't function properly. I have witnessed random NPC dialog between all of the party members while moving around the areas. In short, even in the biggest battles i.e. outside the goblin keep where there are 20 or so goblins, on multiple levels attacking the 5 NPCs and my character; there was very little lag, no game crashing problem, or even party NPCs being ignored by hostiles.
During my latest run through with a full 6 member party- All have hit level 5 before heading down to the Underdark after saving the grove, so Party XP does not seem to be impacted.
