I disagree that other planes have any worse cockpits than the P-51D. If I remember correctly, the P-51D was the very first plane ever made in Aces High (That's why it's number 1 in the plane type listing)
I do agree that the Aces High "cool factor" is lacking when compared to many boxed games. Better graphics and improved sounds could really help out I think.
So far as having customs sounds available, that's fine for those you have been playing and know about them. I think it would be better for HTC to put the best sound files in an 'optional downloads' folder on the web-site, so people who were just getting into the game could get the good sounds without having to look for them.
IMO, AH sounds are not top notch, even when you have many customs. There just dowesn't seem to be the depth of sound you get with other games. Arcade like or not, louder, more intense sound effects would be more fun.