There's a reasonable amount of evidence to suggest that the decision to put a Merlin engine into the P-51 actually happened at the AFDU at the then RAF Duxford
Yes, a test pilot (whose name escapes me) who had been carrying out comparative trials between the Allison-engined Mustang and a Spitfire wrote a recommendation for a Merlin-engined Mustang, though the people at NAA had thought about it as well, and trial installations were carried out independently in the UK and the US.
Edit: c/o Wikipedia
In April 1942, the RAF's Air Fighting Development Unit (AFDU) tested the Mustang and found its performance inadequate at higher altitudes. As such, it was to be used to replace the P-40 in Army Cooperation Command squadrons, but the commanding officer was so impressed with its maneuverability and low-altitude speeds, he invited Ronnie Harker (from Rolls-Royce's Flight Test establishment) to fly it. Rolls-Royce engineers rapidly realized equipping the Mustang with a Merlin 61 engine with its two-speed two-stage supercharger would substantially improve performance. The company started converting five aircraft as the Mustang Mk X. Apart from the engine installation, which utilized custom-built engine mounts designed by Rolls-Royce and a standard 10 ft 9 in (3.28 m)-diameter four-bladed Rotol propeller from a Spitfire Mk IX,[16] the Mk X was a straightforward adaptation of the Mk I airframe, keeping the same radiator duct design. The Vice-Chief of the Air Staff, Air Marshal Sir Wilfrid R. Freeman, lobbied vociferously for Merlin-powered Mustangs, insisting two of the five experimental Mustang Xs be handed over to Carl Spaatz for trials and evaluation by the U.S. Eighth Air Force in Britain.[17] The high-altitude performance improvement was remarkable: the Mk X (serial number AM208) reached 433 mph (376 kn; 697 km/h) at 22,000 ft (6,700 m), and AL975 tested at an absolute ceiling of 40,600 ft (12,400 m).[18]