I just finished the (unmodded save infinite stacking and some other ease of use components) game with TOB and did all subquests that I can think of. The 6 characters ended up with just over 6 million XP each. I only did Nalias and Keldorns quests in C2 and saved everything else for Chapter 6, and I also held off Watchers Keep until after having eliminated the tall Bhaalspawn (as I figured it makes more RP sense to go there when you dont have to rush to save a city that might fall soon).