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.