Hurm, few things:
1) from modding, there is very little evidence that there was some super duper realistic version of the first game ready to go. Every file you see in there other than what's used already (car specific suspensions for example) looks even more beta/rushed than the ones they actually shipped and is even less functional if it's turned on. Pretty much all of the old data you find is like that.
2) They have said the FFB code is all new in Shift 2.
3) Shift never used an unaltered ISI/Gmotor style engine, the tyre model was all new, the FFB was new, surface contact (road/walls/etc) was all new. A lot of other stuff other than that was the same though
4) the issue was never really with the engine - which was a big leap over even most PC sims in a lot of ways - but with the data, and with the terrible job done on testing, which combined to make bugs about 2-3 times as large as any individual buggy component might be.