P-51s and P-47s are excellent over 25k. Those two maintain much of its original maneuverability, and are very fast too.
I dunno why it is so, but the Ta152H-1 sucks over 30k. The speed is faster than any prop plane in the game, but even the slightest bank, turn, changing pitch eats away at E at a tremendous rate and destabilizes the plane, therefore, fighting something at 30k in the Ta152 is absolutely horrible experience.
I've met some P-51s and P-47s at that altitude, and the maneuverability sucked so much that I actually had to drag them down to 18~20k to kill them. Their skill? Let's just say they weren't exactly 56thFG or 412th Braunco material, and I still had a very hard time staying alive at 30k against a P-47 or a P-51.
Spit9 performs very well upto 30k, too. It is the fastest at that altitude, and surprisingly, its turn performance doesn't feel much hampered. At 30k, after the Spit9 and Ta152 takes a HO pass, by the time Spit9 completes its reversal the Ta152 is not even half way through and you can hear stall warning already.
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Contrary to popular belief, 109s suck at high alts. Their reputation for high-alt performance was in the 1940~1942, where "high-alt" was defined as something like 14~18k. SpitIs and 5s would drop performance over 15k...
But when it comes to really high altitudes over 25k, even the G-10 begins to wheeze heavy breaths. At that altitude, I'm not entirely sure that I can beat a total newbie in a Spit9. I'd have to drag him down to 15k or so to get the real edge I need.
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So, in my experience, the best high-alt fighters in AH are the P-47D, P-51D, and Spit9.