Don't forget it was achieved running at 72" Hg at 13,275 lbs. (The details are important
)
Oddly the weight isn't mentioned though, unlike the chart for the P-47N running at WEP where the weight is 13,962 lbs: (Here 20,000 ft is reached in 5.9min)
P-47N M performance at 42.5", 54.5" and 72" Hg.