As long as you're moving your GPS is sensitive enough to know that you're U-turning. It works just fine on my PDA which doesn't even have an accelerometer. There's one in your TT One though. [EDIT: Actually I can't confirm this. There were some GO models without an accelerometer, and I can't at the moment say for sure that there's one in your One. In any case you don't even need anything else than just the GPS for turn-by-turn (or U-turn) navigation. My PDA tells me when I'm at the correct part of the roundabout to leave, for example.]

Other's will have to answer the 2D vs. 3D question though, but in general there's not many navigation applications around that are up to TT standards