The Griffon had similar power at 20-25,000ft with 100/130 grade fuel and +18psi boost (67inHg MAP) as the V-1650-9 did with 90inHg MAP and ADI.
The Griffon-Mustang discussion was occurring when the Mustang X and P-51B programs were in their initial stages. The Griffon Mustang would have predated the P-51H by a year or so. Certainly the 100 series Merlins (of which the V-1650-9 was one) had yet to be developed at that time.
The Griffon Spitfire (XIV) was 30-40mph faster than the equivalent Merlin Spitfire (VIII), depending on the sub type (F, LF, HF) and the Merlin variant used (63, 66, 70). Certainly the Mustang was as fast as the Griffon Spitfire, but it was quite a bit faster then the Spitfire with the same engine.