My thoughts as the p39's are not boosted so high alt is out of the question. For the first week you'll see them all the time then after a month or so they will as common as the B5N in late war. IMHO the D will be a better low alt turn fighter.
I think it will be very popular for a long time. If high-alt performance was the be-all-and-end-all
no-one would fly the La7.... Yet we see that very often.
I doubt it will reach the popularity that the La7 enjoys, particularly now that it has been updated too, but it will definitely be a popular aircraft in the MA given that it is absolutely perfect for MA style fighting.