I can't recall but have a feeling it's around 25mph judging by the speed sign I pass before the descent, I feather the brake mostly to slow me down enough to resist the pedals a bit.
Cadence are personal, generally aim for 90rpm would be a good start.
It is a fixed wheel drivetrain after all, I'm on 45/18, and that was just about spinny enough on the descent in Surreys (66GI).
Stick to the 17t you originally selected, 1t is quite a big jump on the rear.