As far as I know the AP1's a solid platform. About your preference to hardtops, I share it. However both the S2k and Miata were designed as Roadsters, not coupes. Meaning their true design form actually has NO top (Rag or otherwise) at all in it's pure form. Adding a drop-top is a convenience that makes the car more Daily-drivable. To that note, the way roadsters are built is ultra-rigid cross-membered frames. You don't need to worry about frame twist any more than you'd have to with a similar size/powered coupe.
They do however make hardtops for the S2k, that are just replacement removable hardtops that fit down where the ragtop would be. It doesn't really add more structural support, but it also really doesn't need to. If you were going to drift it, I'd tell you you'd need a roll cage to fit underneath the hard top, but you can do that if/when you decide to, though that's a bit extreme. I would love an S2k personally, so here's to hoping you find hapiness with it!![]()