"Edit: Even below critical altitude the Spit 16 would outpace the early P-51 variants"
Not from the #s I have seen. It was not a slow fighter, its just that its high alt performance (above 15k) was more limited than the merlin Mustangs. The P-51A would hit @ 400 mph at 10k, which is a good 20 mph faster than a Spit XVI, whos crit alt is 20k, and its S.L speed was @ 340 mph. It was kept on as a Recce fighter into 1944 in the ETO, largely because of its fast med-low performance. That coupled with its lighter weight (@8000 lbs), it would be a very slippery opponent below 15k, being perhaps 10mph slower than a P-51B but with a lighter wing loading.
