if your in the middle of bg2 and havent beat irenicus and you install ToB is it possiable to run around killing things and get all 8million xp before you kill him and go on to the new part of BG2?

I dunno about that... I'm pretty much stuck in ToB.. i mean.. stuck to the game to stop exploring SoA... I know you can access Watcher's Keep from SoA... but if you have the exp cap there... you'd be wasting all that good exp you can get from the keep..

Yes, you will have bigger XpCap.