I don't know how it will change the balance of gameplay for the overall event, but the Spit8 is far superior to the spit9 as modeled in-game. The raw horsepower gives it +20mph top speed, greatly improved climb rate, acceleration, all while retaining nearly identical turn radius, but with a longer flight range (wing tanks).
IMO the 8 is superior to the 9 in every way, with pure turn performance being so close that pilot skill is the deciding factor.
Again, how that changes the planeset balance I don't know, but I'd guess it would only get better for the allies.
On the other hand, if the writeup is to be believed, the CBA was the arse-end of the supply chain, which is why it has -51Bs instead of Ds, -47D-25s instead of D-40s, so it's not too hard to think they'd have Spit9s instead of 8s, but I totally see what you're saying.
EDIT: Turns out the 8 beats the 9 in turn also, without flaps, by a large margin. They are almost equal with full flaps, but you use flaps in a spit and you're already lost