Go 7 first, then 8 later. The layout is just big enough for an 8 if you keep momentum up. Too small for 9. I think 9 is only good with an aluminum mount for heat dissipation.
I think a 9t is not really faster than an 8t in anything but a very high flow track. If there are any slow sections, the 8t is ideal. There is a little heat fade with an 8t, but nothing compared to the 9t. With a 9t you need to baby the throttlein the early minutes to have any power near the end of the run.