So lack of air resistance would facilitate a much higher top speed, but would starting off be noticeably easier (ie could you use much higher gearing than usual). Or is that more to do with overcoming inertia?
The latter. Gravity only really works against you when climbing, and there's none of that in a velodrome.
Air density is why velodromes high up in the mountains are considered "faster" than those at sea level. The minimal difference in air density does make a difference at these speeds.
So lack of air resistance would facilitate a much higher top speed, but would starting off be noticeably easier (ie could you use much higher gearing than usual). Or is that more to do with overcoming inertia?