From what I've read in IA, it seems to play out as;
Tau pilots are biologically better for air-to-air combat, able to pull off maneuvers that would result in a human blacking out. The Barracuda, the standard air superiority fighter of the Tau, is far more maneuverable than the Lightnings and Thunderbolts used by the Imperium.
While the Tau have the edge in biology and maneuverability, the Imperium on its side has experience and speed. Imperial Navy pilots tend to learn fast or die, so they'll quickly gather a hard core of very experienced, skilled fliers which the Tau struggle to match. Their aircraft are also faster on a straight than the Tau.
Technology is difficult to gauge. Tau technology is certainly highly advanced, but Imperial technology is rugged, reliable, and good for blowing huge holes in things. They've both got advantages and disadvantages and it's hard to really grade one above the other. One advantage the Imperium does have over the Tau is its fighters can also be used as limited bombers, with clusters of bombs under the wings. This makes them slightly more versatile, as they're better ground-attack aircraft than the Barracuda is.
When it comes to numbers, the Imperium certainly has the advantage. Overal I'd say in a one-on-one fight, it's pretty much equal. But the Imperium has the manpower that allows them to make sure it isn't a one-on-one fight.