Not a flame but - As its the D9 you must be talking about 1944.
I don't think the IX will get much use (if any) once the XVI/XIV becomes available in 1944.
At alt the XIV is far superior to the D9
Up to about 20k the XVI has around the 3rd best climbrate in the game, behind the K4 and the XIV.
I think someone else worked out it has the 2nd best acceleration, La7 only just shading it.
Wouldn't advise a climb unless the D9 has a lot more 'e' than the XVI, and wouldnt advise a climb against a Spit XIV period.
Best bet is to stay fast, but then you have to deal with the Tempests (fastest prop plane in the game).
A good 1944 RAF flight will hopefully have mix of XIV, XVI and Tempests.
Pref XIV's top cover, Temps around 18k initally, XVI slightly below them.
XIV to deal with high alt cons, Temps to chase down and turn cons, XVI to mop up.
Personally the only plane I will be worried about is the 262, but as long as I see him 1st, I'll be OK.
1944 going to be very interesting.