Not meaning to be disrespectful, but I don't think the Spitfire Mk IX ever served in the Pacific or CBI theaters. The Spitfire Mk V, Spitfire Mk VIII and (not in AH) Seafire Mk III were the versions used in those theaters.
I don't think the Mk VIII would be unbalancing either as in prior matchups it came off badly against the Ki-84.
Yes, I plan on flying some sorties in the AvA during this setting, probably in Ki-43s, Ki-84s and A6M5s.