Ah, here's my confusion: I had previously looked at the stats as-modeled in game and they were identical.
The speed charts are on top of each other for the D-25 and the D-40. Only, the D-40 has an ever-so slight increase in climb rate on WEP. I thought it might be a weight issue, but now that you mention this I can see the horsepower being the reason.
EDIT: I can understand putting the D40 in... It sets them apart performance wise and weapons selection-wise. Otherwise we'd have 2 identical planes, one with and one without a fillet.
If that was the case, we'd have had almost total universal changeover to P-47Ns in the AH jabo community hehehe