Those headrest hoops are usually cosmetic. If you take a Miata to track you better invest in a roll bar and proper race helmet, head restraint, and harness.

you need both the A-pillars and (effectively) a roll bar behind the seats in order to protect the passenger area from getting crushed. With weak A pillars I think a rollover will fold the windshield down onto the occupants and possibly crush them into the seats, depending on impact angle