I wonder how many of the performance issues are just bugs that are easily fixed once tracked down and how many are limitations of the engine and the way it was designed to utilize hardware which would require extensive work and re-coding.

I think that DX9 compatibility was probably one of their biggest mistakes. Wasting so much time and effort to make the game compatible for cards that are too slow to run it, and an OS that Microsoft has pretty much abandoned must have eaten up so many man-hours that could have been used to make it run properly on DX10 and DX11 (which are so much easier to code for).