The Spit VIII is almost as good, and is in fact my preferred Spitfire.
But as good as the Spitfires VIII and XVI are, the last kill I had in AH before I canceled my account was a Mk VIII that bounced my Mossie just as I killed a P-51D. I knew the Spit was coming in (I guessed it was a Mk XVI, just playing odds, when its icon first appeared) and I hurried my kill of the P-51D, as soon as I killed the Pony I pulled a hard break turn at the same moment the Spit opened fire and I hear the light crunching sound of .303s hitting my Mossie's wooden body and the chatter of his cannons firing as he zooms by. I knew as soon as I heard the .303s that it wasn't a Mk XVI, but regardless in the next 30 seconds I took a fully defensive Mosquito that was out of E and annihilated a Spitfire Mk VIII that started with lots of E.
My final sortie in AH resulted in the kills of a Panzer IV H (random kill, spawned under my bombs as I dumped them to engage the La-7 that was inbound), La-7 (came in co-alt, but fast, was out maneuvered and killed in a high deflection turn), P-51D (started as a base porker hitting the VBase I was near, then started attacking me with an initial alt advantage of ~5k, I bled his E out and killed him in a turn fight) and the Spitfire Mk VIII (out maneuvered him through about three full turns where his E bled out and I held what E I had in high yo-yos, and killed him as he leveled off and tried to run from my Mossie).
So, we had three of the most common fighters (Spit XVI would have met the same fate in this guy's hands) against a fighter-bomber in a quick series of 1v1 fights and they all lost. Perk the Mosquito? No, that sounds silly. If I had been in any of those fighters the Mossie would have died, particularly the Spit which got to start with a huge E advantage when the Mossie was nearly out of E. I am not a very good AH pilot anymore, very rusty due to inactivity and I was never great, just above average at my peak. Why did I win? I knew my aircraft and I used it to its advantages while they probably, and fatally, thought I was a free kill.
I do toss a .salute out to that P-51 driver who started as a high speed porker, but decided to actually try a fight rather than just auger or run.
Don't why the 16 spit lovers don't fly the 9 more often, great little crate.
Because it is too slow in the late war arena. If the fights were at 20k+, great, but not as AH altitudes. It is also not very representative of the majority of historical Spitfire Mk IXs. The Mk VIII or Mk XVI is much closer to those.
