The P-47 also had better performance at high altitudes and was better to ditch. Grabewski seemed to do pretty well with it in aerial combat, so I think the idea that it had the general performance of a brick is nonsense.
Strictly speaking the P-47 had very slightly more speed above 33,000 feet. Zemke was a superb leader and quickly capitalized on the P-47 performance above 25,000 feet compared top FW 190 and Bf 109. So, his planned escorts were at 28,000 give or take but nominally 4-5000 feet over the bomb wing the 56th was escorting.
No other P-47 equipped fighter group in any theatre scored as high as 50% of the 56th FG.
Agreed regarding the 'brick' comment. Excess Hp to drag was king at very high altitude and the turbosuperchared, water injected R-2800 availed the P-47 better turn than everything else until Ta 152 above 31,000 feet ( the P-51B tapered off above FTH of 29,000 feet, but climb performance compared to P-51B was about the same due to cooling drag delta.
That said, veeeeery few fights above bomber altitude.
However, it was generally a moot point in the run-up to D-Day as the P47D had to turn back near middle Germany while the P-51B/D was fighting all the way to Posnan, Warsaw, Vienna, Ploesti.
At the Patuxent River Conference where all US and some RAF fighters were flown by USN,AAF, USMC, RAF and civilian pilots the P-51D was rated best under 25,000 feet (FTH of P-51D V-1650-7 was 24,000 feet) and the P-47D did beat the P-51D for Best above 25,000 feet, but only barely.
Last, but not least, Zemke is on record as stating that the P-51D was better than P47D and P-38J in combat - and he flew and scored in all three.