The 51B is in a lot of CT sets, while the 51D is rarely in them, that is why the Rhine set is nice, it allows all those really late war planes to be used. Likewise the Spit 9 is in tons of sets, it wont hurt anyone to leave it out for one week in exchange for the Spit 14.
The Spit 9 wasnt neutered, the Spit 5 was given 16+ boost where before it had 12+. This makes the Spit 9 redundant beacause on the deck the Spit 5 is only 2-3 MPH slower than the 9 but turns substantially better.