I had a sweet high-alt P51B vs 109F4 duel not long ago. I thought it was G at first, until he kept up with me. I had to turn slow, as not to lose speed. We were 15k or more. So because we both were trying to keep speed up (~200) we extended a lot, but when I pushed it, and when we slowed down, that 109F4 kept up with me in flat turns. I made some mistakes and lost a flap, an aileron, and my oil, but it was a great fight.