I believe Doolittle asked the British RAE to assist him with one thing - to investigate why the USAAF fighters of the time (P-51B, P-47C and P-38H) couldn't match the Luftwaffe Fw190s and Bf109s in very high-speed (of the day) maneuvres. The speeds where the control difficulties occurred where - at the time - only achievable by piston engine fighters in a high speed dive. The Luftwaffe's preferred tactic with the Bf109 has always been a bounce dive approach from above - with it's relative small size and high performance and heavy firepower. Against escorted USAAF bomber formations, it was more a necessity, so as to reduce exposure time to bomber return fire and escort intercept. It would appear that under these conditions, the USAAF fighters could not catch the Bf109s/Fw190s in a sustained, controllable manner. Doolittle wanted to know why. Brown and his fellow RAE pilots demonstrated to him why this was the case.
This commentary - taken out of context - has been used to denigrate Eric Brown on a number of occasions. The P-38 comment has to be viewed in light of what the discussion was actually about. NOT that the P-38 was useless as a fighter, but that in countering the high-speed dive attacks by Bf109s/Fw190s, the P-51B was more suitable than the P-47C and the P-38H.
There is no dogfight detail if your P-38H loses control when chasing a diving Bf109 at high speeds beyond Mach 0.68.