From the read-me for Patch 6.
Fixed a bug with the P-47D-30 that was causing it to not get its full WEP performance.
W00t!
See Frenchy, your tests and information eventually paid off! OK, it took a while, but it did get fixed. Thanks Frenchy, for testing and sending Pyro the results. Hard facts get results.

It's not time for me to go back to the D-30 I think.