From what I can find, the short span ailerons on the VIII were added at the same time as the wing was strengthened. There had been a number of accidents where earlier Spits were lost in high speed dives where aileron flutter was considered as the reason for the accident.
The shortened ailerons were to reduce the possibilty of aileron flutter. Nothing seems to indicate there was a reduction in performance because of it.
They were introduced on the high alt Spitfire VII first and were also on the Spit XIV but interestingly not on the Spitfire PR19 which was the recce version of the Spit XIV.
As far as AH is concerned, I've been bouncing between the IX and XVI and I don't know that I don't like the IX a bit better. It just feels 'smoother' and in fights against XVIs, even down low, I've been able to knock em down.
Nothing to back that up numbers wise, just the feel of the IX vs the XVI.