Looking at P40 Speed Performance Numbers...

[secretone]

So there was not a huge increase in performance in the F model because the blower was only single stage. Apparently Packard was not yet producing the 2 stage version of the motor at the time the F model was in production.

Here is a discussion thread for anyone who wants to explore this further:

http://www.ww2aircraft.net/forum/avi...40s-30910.html