On the contrary, the radiator on the spitfire could actually provide positive thrust, simply because it took in cold air at the front and put out hot air at the back.
Well, yes, but for the same engine with the same waste heat removal, a bigger radiator doesn't help. It would be worse, actually, since you have the larger drag, and the Meredith effect would also be smaller since you'd have a larger volume of air passing through, but heated less.
https://en.wikipedia.org/wiki/Meredith_effect