bozon: While you are correct about excess power, if you look at most planes the drag ,not do to weight change, at climb speed is a fairly minor change in most planes of the WWII era.
And typically you can rank most planes climb performance with nothing more than HP and weight.
If they do not rank that way , it typically has to do more with propellers than airframe.
Note all this is rule of thumb stuff, and not precise calculations.
HiTech