The 645s are a good strong servo but on anything short of a gaser I prefer a faster servo (the 645s are pretty slow).

I find it harder to do some maneuvers such as rolling harriers when using slower servos on the tail.

People are talking about having a "locked in" feeling when flying 2.4GHz because of the lower latency -- and we're talking about a difference of around 20 thousandths of a second.

The difference between a fast and a slow servo is as much as a tenth of a second or more -- you can see what effect that might have.