Personally I reccommend a Buick for track events.
Nah, Pontiac Grand Am FTW:
It doesn't have that racey name for nothing...

Yes, it's high centered. Just lean forward and it comes off...
I've run rentals (as most of you know) at many, many, many events. As long as you don't go shouting to the world that it's a rental, most don't say anything.
Rentals generally aren't fast enough, nor have good enough tires to boil the brakes. Plus, since they're not your tires, you can always just trail brake instead to scrub speed.
Stay away from the Mazda 6 (or probably any automatic Mazda). The damned near self-aware computer would determine I was abusing the car too much and take away the manual gear select function and shift straight to 6th. This is a major problem when the first time you discover this is going really hot into turn 8 at Willow Springs. Nothing like 100+mph 4 wheels drifts with no cage, a wall close enough, and no way to correct it in time with the gas pedal. Yikes!
Oh, I would also disable the ABS. If (when) you go off track, ABS is nearly worthless in the dirt (read: NEARLY NO BRAKE APPLICATION WHATSOEVER). Since you're always told to go both feet in (well, one foot with an automatic) when you go off track, this can get, err, exciting?