drgondog
Major
The Hornet design came from early studies on the Mosquito being looked at as a long range escort fighter.
The results were that it couldn't, but they did some trials with a (very) lightened Mosquito and the performance was remarkably better, but not operationally practical.
Basically the Hornet was to the Mosquito what the prototype super lightweight Mustangs* were to the P-51D (with their also remarkably better performance).
*Note the P-51H was not one of those, it was only slightly lighter and the real rational for it was the ever decreasing G limits of the P-51 A, B D (each model going down) especially with high fuel loads.
The H restored those even with high fuel loads and its higher performance really came from using a 100 series Merlin.
True the F/G/J Mustangs were even lighter (~600 pounds) than the H, but the H was ~1000 pound lighter than the D/K and 500 pounds lighter than the B/C - but carried 6x50 cal vs 4x50 for the B/C
The 1650-9 engine on the P-51H was basically the 1650-3 with WI added to a modified pressure injection Simmonds boost control.
The Rolls 14 S.M. was in the last two of five XP-51F's and re-named XP51G - and this was best performer with 495mph at 26,000 feet and service ceiling of 46,000 - only because cabin not pressurized to enable pilot to climb higher. Having said that, when you added guns and armor and external racks it would nver have climbed to 20000 in 3.4 minutes, made 495mph or climbed to 46000 - it would have dropped closer to P-51H capability