MiloMorai,
That is at a higher altitude than Mosquito VIs operated. That said, I've never heard that P-51s had a problem escorting Mosquito VIs. Worst case I can see is the P-51s have to run at a higher setting than their normal cruise.
Spitfires on the other hand had a hard time escorting Mosquitoes. One Spitfire pilot told me that the dumbest mission he ever flew was escorting Mosquitoes. He said the Spitfires had to fly at full throttle to keep up with the Mosquitoes. He flew Mk Vs, Mk IXs and Mk XIVs, but I am not sure which he did the Mosquito escort mission in. I got the impression it was in a Mk IX.