I took the same tour a few years ago. I asked our guide about what the "average" gratuity was and she simply said "it depends". Our guide also led Tony Stewart on a VIP tour a few weeks prior to our visit so I think her response was probably appropriate. You'll definitely want to tip something, and I would think that a reasonable tip would start somewhere between $50-$100.
A couple of things I'd strongly suggest doing:
(1) Don't try and power ride the full eight hours. There's plenty of time to get on what you want to get on. Take a few breaks to take in a show (I'd recommend the On Broadway show), cool off, snack, and stay hydrated.
(2) You get reservations to restaurants and shows. Utilize the hour break during your tour to eat at a sit-down restaurant. Utilize the host to reserve any shows you want to see so you can get the best seats.
(3) You get early entry privileges, but my advice would be to pass. It's a long day and you'll already be doing a lot of riding. You'll appreciate the extra sleep you gained by not getting to the park that early later on that day.
I won my VIP tour as part of a contest and it was - by far - the best day I've ever had there and I wish the same for you. Have fun!