I'm not a good pilot, but there are some things I've noticed while fighting at high alts. The air is thin over 20k, and the plane picks up speed much quicker than when you would be fighting under 15k. A slight dive of 1000~2000 feet will usually send the fighters with high speed capability (G10s.. Doras.. and especially the Mustang!) soaring over 380mph easily.
Also, while speed picks up faster, the controls lock up easier.. I usually fly the G6 and G10, and experiencing heavy controls under combat conditions is not uncommon. I think this is why planes such as P-47s and P-51s are superb.. They are just as fast as most LW fighters, but high speed control remains very stable even near compression point

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High alt fights are almost all about speed and roll, rarely does pure 'turn rate' come into consideration.. how one manages to keep his speed optimum, while still controlling the plane enough to roll inside the enemy(thus, the 'rolling scissors fight') comes more important.. just like Tumor put it. And at this point.. the Mustangs and Thunderbolts shine out.. So does Dora up to a point, but the roll rate seems to drop slower when they fly over 400 mph.
The scary part is, the P-51s have their uncanny ability to bring the flaps into action.. giving a sudden edge over planes such as Dora or G-10.
And, like I said, I'm just an average pilot. Therefore, this is where flying G-10s become uncomfortable for me... especially when one faces Mustangs up high. Therefore, I try to keep the fight all vertical, only reliable factor being the superior climb rate. Just pass pass pass around and eventually gain higher alt than those 47s and 51s.. just like some people mock about

- I just pendulum around.
I understand things would be different with planes with better high speed control such as 190D-9s, P-51s and P-47s. I haven't flown the Dora enough to find out, but I'm betting there is more of 'action' when fighting up high, than flying G-10s. Probably more agressive roll fights occur. (Just the kind of fight a 109 would hate to get in

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Also, another thing I noticed is, when fighting at high alts, you can't regain alt as fast as you do at low alts. If I dove a G-10 from 15k to 5k, I could rocket up back to 15k and still have enough control and speed with my plane to continue the climb with shallow pitch.. but if I dove from 27k to 20k, I climb back to about 24~25k and already my plane is near stall point. So, high alt fights are usually down hill fights. Fights among more skilled pilots would be a shallow down hill, while fights among average people like me is a deep down hill slalom

And, final factor I've noticed is, high alt fights either a) end very quickly, or b) end up with no winners. First 5~10 passes or so and who is more advantageous becomes clear, and the disadvantaged pilot chooses to run. Jut dive to nearby friendly base end the fight is over. Of course, both P-47s and 51s are able to catch runners in pure terms, but rarely do they do so risking losing their alt and quickly being surrounded by other planes nearby.

Hope this piece of info helped.