Given the sim I've played before, an arena full of Spits wouldn't be anything new to me. In AW about 2/3 of everything in the air a lot of the time would have been either a Spit or 190A8.
Regardless of the statistics, there is a great deal of variety in AH's arena. Granted some planes are more common than others--this is inevitable--but no one plane dominates the arena. I see plenty of Spits, 109's, LA's, P-51's, F6F's, N1K2's, 205's, and so on. About the only plane that seems under-represented is the F4U (the F4U always dominated AW's pacific arenas).
Furthermore, no one plane seems to have a dominating advantage, now that the N1K2 lost its "rocket boost". If the Spit IS the most common, I have no problem with it because it flies "right".
J_A_B