There are many 'Spit IXs' with different engines.
The Spit XVI as modeled in AH and as the pictures reflect are of a Spitfire LF.XVI with a clipped 'E-wing'.
As such its engine would be a Packard-Merlin 266 (Ami built Merlin 66).
The Spitfire MK IX in AH is a Spitfire F.XI universal wing (2 x 2cm Hisso's, 4 x .303s) with a Merlin 61.
Max boost for the F.IX Merlin 61 is 15 lbs
Max boost for the LF.IX / LF.XVI Merlin 66/266 is 18 lbs (assuming the modeled the LF.XVI @ 18lbs; with 150 octane fuel it could be run at 25lbs).
However, the LF is optimized for Medium altitude. Some say low altitude but that's not actually true. Between 12 and 20k the LF will easily outperform the F.IX. None of the Spitfires in AH have great deck speed, including the XIV. The F.IX will still be faster the higher you go. With clipped wings the LF.IX/XVI with roll better then the F.IX. Climb and acceleration will also be better (marginally depending on the altitude).
EDIT:
oops was typing when karnak posted...