Virgil,
The Merlin 66 with it's lower altitude performance was a direct result of the tests of the high altitude Merlin 61 Spitfire F.Mk IX's tests against a captured Fw190A determining that the critical altitude on the Merlin 61 was almost 10,000ft too high. The most produced Spitfire Mk IX was the LF.Mk IX with the Merlin 66, though they also made about 1000 HF.Mk IXs with the high altitude Merlin 70. The definitive Merlin Spit, the Mk VIII, was mostly powered by Merlin 66s are well.
It really depended on the role the aircraft was being used for. High blown engines were more desireable for P-38s, P-47s, P-51s, B-17s, B-24s, Lancs, Halibags, B Mossies and NF Mossies. Spits were more often employed for fighter sweeps, small scale strikes and medium bomber escort into France and the low countries, for all of which it was desirable to have engines with a lower critical altitude.
On topic:
I had no problem fighting La-7s, they just seemed to have problems wanting to fight me. Fighting was never the issue.