The Spit IX is a very very good plane, - in my opinion the best overall fighter in AH. There is no plane both faster and more nimble. And it's well armed too.
It does have inferiorities in some fields against certain planes, but as well superiorities against the same, so duelling a Spit IX on equal terms with another plane usually leaves you in the mud, - if the Spit pilot knows his job.
The main weakness of the Spit IX is speed, - many planes in AH are quite a bit faster, so the B&Z is the favourite of many. however, a proper B&Z requires something else than equal terms.
And imagine, the Spit IX we have is not nearly the best Spit, its just a modest high alt fighter from 1942.....