The 190a5 performs very, very well in the vertical. It's good roll rate allows it to avoid giving a guns solution to planes no matter how close behind they are, and its controlable stall means it can keep doing that right up until the end. Basically, if your opponent isn't in a helicopter like the P38 the a5 can reverse it in the vertical. The a8 can only hope the guy in back doesn't know how to scissor or cut throttle.
This huge performance gap doesn't make a difference in the MA, however. Most pilots are far too reluctant to go up when someone is on their 6, for fear of the increased energy bleed giving up any angles advantage they had left.
-pellik