The hardtop will not be any problem at all.
But if You're Mick Harrison, I thought you had a roll-bar???? You can run top down. They don't do any broomstick stuff for AX and TT...only at DE events. Just a good ol Hard Dog or other full loop bar. If it looks like a style bar you'll be hassled...since they don't let their Boxster drivers run top down with the factory double hoops.
Look forward to seeing you and the other 6 Miatas registered for the school on Saturday, only 4 on Sunday ;-(
Maniac Mitch