$90 sounds like a steal to me if the seat time holds true and the fact that I can save myself sunburn by not working the corners

This is only slightly more than what you would pay for 10-15 mins of total combined seat time over the course of 2 typical autox events.

The "grip drivers" would probably like some sort of timing to be in place to measure there improvements. Will rollbars be a requirement? I know we have several convertible owners S2000/Miatas etc that would be interested but not all have a rollbar.

How many "open slots" are we talking about?