Yes, many people have been able to develop end-game characters which are much more deadly than anything we made during balance testing. We'll try not to make the same mistake twice.
Although I guess the tricky thing is to do so without making it impossible for less powergaming players to also complete the game.
When building for NWN, I always found it quite tricky to create areas which presented equal difficulty to both players who build to a roleplaying concept as opposed to game advantage, and also to the sort of player who's quite happy to take a single level of paladin with their sorcerer for the saving throws (the poster-child of 3rd edition D&D powergaming goodness).
Of course, that was a multiplayer server, so it was easier in a way - there was no "central quest" that players could expect to complete, so it was easy just to throw in some absurdly hard quests and powergamers could attempt them at lower levels, or roleplaying characters could try them later.
Mind you, I guess you can do the same with a single-player game - if people want to create focused, powerful characters, they'll be able to complete the main quest at a slightly lower level as their "reward". Alternatively, less "strategically built" characters can level up a bit more and get through that way.
Either way, I'm sure you'll do a great job, BW, I was just musing on the subject for interest's sake.