I think it is a combination of a number of things:
Lighter, weight balance moved forward, extra diff ratio, modified gear ratios, changed suspension, and increased the maximum revs.
Not only is the speed improved but also the handling, but it is now at least 4 to 6 secs quicker per lap at most tracks, making it potentially the quickest car in the TC-65 catagory, which I love but don't really think is realistic.