The F4U-4, in terms of overall performance, is probably THE best American fighter in the game, if not the best of ALL the prop fighters. I've flown her from time to time, and I'd fly her MORE if it weren't for the perk cost. To think of it this way, the mid-war F4U-1, excepting acceleration and rate of climb, can hang on relatively equal terms with most any of the late-war birds. The F4U-4 is even BETTER. I don't know what complaint you have about the -4's acceleration, but she's an absolute rocket, ESPECIALLY compared with the -1, Charlie and Delta, capable of a sustained climb in a range of 3-3.5k/min to 10k alt.
And the reason the 51D should be perked if the 4-Hog is perked, is 1) BECAUSE she's used so much (especially by those chicken-***** porking Runstang dweebs) and 2) She's fairly close to the F4U-4 in performance, with a slight edge towards the Hog. Close enough that I think it justifies a perk SO LONG AS THE 4-HOG IS PERKED. So either the 51D should be perked, or the perk should be REMOVED from the F4U-4.
And as Widewing suggested on another forum, any concerns about acceleration in a 1, C or D-Hog can be addressed by entering the fight at higher altitude and pointing that big nose at the ground. Depending on the situation I try to arrive at between 10 and 15k alt.