if Gonzo's chart is correct, A8 is faster than the A5 (Wep) from 0-5k, 10-20k, acceleration of the A8 is faster from 150-300mph at the deck.
I've read the charts and checked the charts with my own testing. Mind you, what the charts say and the actual performance is can be two different things. Double check the weights of the plane as registered on the charts, then hop in to the MA and test it for yourself. Speeds especially are not exact.
Without testing it outright, I'd put the A5 ahead of the A8 at all altitudes in terms of non WEP speed and acceleration. With WEP, they are close enough that a while engaged the differences are not going to matter, ultimately. The A5's ability to turn tighter, climb better, and otherwise provide a much smoother platform with carry it further than losing/gaining 5mph of speed or a split second or two of acceleration (on WEP).
Trust me, I take nothing away from the A8. It is a monster and isn't left in the dust but the A5 is more of a decathlon athlete while the A8 is more of a strait up shot putter. I'd leave it at that.