Hi, Seacue..
Yes, the P-40 grew directly from the Model 75, at least
AFAIK. The original Hawk had some problems with engines, from what I've read. The original Wright radial was changed at the last minute, apparently, for the delayed trials by the USAAC. Unfortunately, the Hawk was slower with the larger Wright. An engine/prop mismatch, perhaps.
There were probably a few other changes in the P-40 but the inline Allison engine was the icing on the cake. You can't rule out the speed-enhancing effect of painting a shark's mouth on it, of course. Like painting a sports car bright red makes it go faster.
Glenn