The Spitfire variants are sub par in performance to the 109 and Hurricane at altitudes above 15,000ft. There is, in addition, a bug with a number of the Spitfires at altitude; the Spit 1a 100 Oct is most severely effected but it is there on other variants - as you gain altitude the engine splutters and cuts out unless you run at reduced rpms and boost below anything like combat settings. I'm at a bit of a loss as to how we've been left with Spits in this state in the final patch.
As a Spitfire pilot at the moment you have to pick your variant on based on what altitude you want to fly which is fecking ridiculous if you pardon my French
Altitude fighting might not be important to those that like to hug the deck on ATAG or Replica4 but it is important for campaigns like the SoW campaign where the fights start at 17-18 thousand feet but often end up going much much lower.