I don't know where this high altitude performance for the P51 comes from but maybe it is a function of definition of high altitude. If you look at a performance graph of the P51D it had a Vmax w/combat power at sea level of about 360 mph which climbed to about 415 mph at 11000 ft, then dropped off to 405 at 15000 feet and back up to about 435 mph at 24000 ft where it starts really dropping off where at 35000 feet it only gets about 385 mph. The P47D had much better high speed above 25000 ft than the Mustang so I would not rate the P51 as a premier high altitude fighter. Of course I doubt if much ACM took place above 25000 ft so the Mustand had plenty of tactical speed for most encounters.
For the purposes of the Mustang's prime mission - namely long range escort of daylight Strategic Bombing - High altitude definitons should be framed around the escort levels of the B-17 and B-24 - namely 20,000 to 26,000 feet plus another 3-5K to engage LW fighters entering with an altitude advantage. The later model 109G/K's and Fw190Ds were designed to perform much better in this arena... but were at extreme disadvantage in early 1944 through end of year at these altitudes.
For the purposes of Interceptor Mission of Recon ships - then 38,000 - 45,000 is "High Altitude". That isn't where the Mustang played but is where the 109G, the Fw190 had to play if they wanted to try to stop daylight recon.
Frequently the excellent performance of the Fw190D9 thru 12's are cited at the higher altitudes, and when the 3 stage supercharger actually worked on the Jumo 213's, their speed was exceptionally close to 51's at the 51B/C/D best altitudes, and slightly higher at 35,000 feet - but irrelevant to stoppping B-17s and Mustangs and Thunderbolts from hammering Germany
Here are the late Brit comparisons found on Williams' website for the Mustang, Tempest, Typhoon, Spit IX, Spit XII, Meteor and P47D for your consideration - you have to be careful on all judgements because the 'controls' for the tests are not well presented per altitude, loads, fuel used, condition of a/c blowers, etc.
Comparitive Performance of Fighter Aircraft
As for the 47D trials for USAAF
http://www.wwiiaircraftperformance.org/p-47/p-47-tactical-chart.jpg
You'll notice that at the war emergency HP the max speed for the 47D-26 was 357mph at 5,000 ft and 425mph at 30,000
For the 51B/C and D
http://www.wwiiaircraftperformance.org/p-47/p-47-tactical-chart.jpg
the corresponding war emergergency HP the max speed for the 51D-15 was 375mph at SL and 430mph at 30,000 feet. 51 still faster basically across the board, turn much better, accelerate faster, initial climb better, slower roll and slightly slower dive..
Point to note - the 47 had to carry much more fuel to go the same distance on a medium to long range escort, making the Jug much more sluggish in acceleration and climb both on the deck, in middle altitudes and at high altitudes.
Point to note - in the first link, the Brit Report, you will note that the Jug was by far worst turning ship in the comparisons but the 51D was beaten only by Spit IX while the 51D beat the Spit XXII..
And the 51H had better actual speed performance at high altitude than both the P-47N and M (and much better than the D) - you will have to look further in Mike Williams site to dig out the details
Take what you want out of this - just food for thought on the most over rated fighter of WWII.
Regards,
Bill