As a comparison, ToEE is pretty extensive and uses turn-based combat, but it certainly doesn't take 500 hours to complete. (Surveys say around 65 hours for completionists.) Part of that may be due to the D&D v3 system, which provides faster leveling up. But certainly the game designers would have changed things up if BG were turn-based so that the combat wouldn't drag things along too much. Presumably that would have meant fewer but more interesting battles.
Guess I'm just agreeing with daveyd here.
BG was significantly larger than ToEE.
I like both types of combat, but I think RTwP worked great for the IE games, and I feel they made the right choice in using it. I couldn't imagine playing BG or IWD any other way.