I use 9's on my seven string, 8's on my six stringers, and im gonna be using a custom set of 8 tops, 9 middles, and 10 (with extra strings of course) bottoms on my 8 string.
The string gauge doesnt really effect how fast you can play i find (well, within reason, i think just about anyone would be slow on 13's). Ease of bending is the main difference you will noitce, and therefore ease of vibrato.