Our B model has a Packard Merlin just like the D but a different model and it peforms better at higher alts than the D. (not sure on the model numbers but HTC did comment on it indicating it was a hi-alt version).
I've flown both and i havent noticed much difference in them, except the B feels slightly more nimble. The B should be faster on the deck and faster above 20K than the D, it should reap the benefits of being slightly lighter, lower wing-loading, slightly better climb etc.
The Aceleration in both still suck ass, and the turning ability is marginal at best, so turn fighting is bad news in either model.