I guess it all depends on what your style is. If you're looking to dictate the terms of a fight, be able to disengage at will, and have a higher probability of landing, then yes, I suppose speed would be the #1 asset in the MA, much as it was in real life.
Those aren't really my priorities. What I like is being able to fight under any terms dictated to me, to be able to stay engaged for an extended period of time, and I really don't care too much if I land or not. For me, speed is just a useful tool for forcing a fight.
I'd say the most valuable assets for me are acceleration and climb rate... I probably feel this way because I spend much of my time in a plane that lacks in both departments, the F4U-1A...
With acceleration and climb rate, I have the opportunity to regain energy that I spend flying like a guy with a deathwish. My style does not usually require an excess of energy over my opponent, but just enough energy to be able to maneuver effectively. In a Corsair, my maneuver window shrinks and shrinks as the fight progresses, and without sky beneath me, it is very hard to reverse that.
I find that I can stay alive longer when fighting multiple cons in something like an La7 or 109K4 than I do in a F4U1-A, despite the facts that I've spent most of my time trying to learn the 1A, and that I personally find the 1A a much better 1v1 knifefighter than either the 109 or La7.
Just look at the F4U-4... It's universally regarded as a monster in this game, yet not because of its speed - all Corsairs are fast... It is a monster because it can do all the things a regular Corsair can do, plus regain energy quicker with a much improved acceleration and climbrate. Its maneuever window does not shrink nearly as quickly as its predecessors'.