Never look back. Now that you've driven a huge honking pig, a Miata will be a delight. I took the 3L SVT out a couple of months ago, and I think that I threw up in my mouth a little bit after the first run, wishing that my Miata was there in its stead. You have chosen wisely; my only suggestion is that you do a couple of events on street tires so that you can get the feel of the car under your belt without relying on the poor techniques that race tires can give you right out of the box. If you can stand it, do Azenis for a whole season. You'll learn a LOT about what the car can and can't do.
Expect to be slower, figure out the low torque, low weight, wheels driven in the *right* place and relearn what momentum maintenence is. I fell into the trap of letting torque make up for momentum slowing and it absolutely killed me when I got into my Miata. But now, I have a third place medal in CSP from the Virginia Commonwealth autocross championship sitting on my wall. Suck it up for a year, trust me, you'll appreciate it.
