Like I said, I can buy the fact that the 9U should be among the fastest planes in the game in level flight. It was light, it was sleek, and it had a fair amount of power. And if it was faster than the 51 at certain alts, I can easily see that being the case. I'm not going to even argue which alt this should happen at, because I have far less data available than those who make the FM
What I do not see, however, it where the 9U gets the ability to run down diving 51s with a head start over a chase of a full sector or more at speeds in excess of 500 TAS at low alts.
When you get into this regime, most planes in the game start having serious compression problems. Their controls lock up, the whole plane buffets, and your speed stops increasing; you know the story there. Most planes encounter the problem in the lower-mid 400s, some in the upper 400s, and a few (basically the jets and the 9U) in the 500s. I'm talking IAS in the typical low-alt MA situation here, where it's pretty much the same as TAS anyway--at strato alts, TAS seems to be way more important as to the advent of compression.
Anyway, with the 9U, the lateness of the appearance of the compression effects gives it the serious advantage of being able to run down every prop plane in the game, even the perks. The 9U retains the ability to accelerate to higher speeds before it hits the wall, and it retains control longer. So over the course of a full sector of chase, it will catch up with the 51 (or any other prop plane) because it's able to reach a higher top speed, and it will be able to maneuver enough to get a shot when it gets into range, while the 51 is locked up. Trust me on this--I've been on both ends of this situation many times.
I just don't think this is right. I would put my money on the 51 having more resistance to compression because of its laminar flow wing, compared to the 9U"s apparently standard airfoil section. Both had about the same frontal area and both were fairly sleek, so I figure you have to look at the airfoil section to come up with the real determining factor. So I would think that the 9U would start getting shockwaves building up on its wings way before the 51 does, and thus have a lower max uncompressed dive speed.
NOTE: I am not talking about acceleration here. And remember there are 2 types of acceleration: on the level and in a dive. The former is where power-to-weight ratio is important, while in the latter, sectional density becomes very important. So other things being equal, the lighter plane should accelerate better on the level and the heavier plane should accelerate better in a dive. IOW, the 9U should beat the pony on the level, but the pony should beat the 9U in a dive.
So not only should the pony pull away from the 9U at the start of the dive, due to better acceleration, but the 9U should hit the compression wall at at lower speed than the 51, so should not be able to catch up at all. But this isn't the case in AH. The 9U out-accelerates the 51 in a dive, and has a higher max dive speed as well, so can always run it down.