In theory I definitely agree that an X across the center of the chassis is a great id
An "X" across the center of the chassis running with the legs running from the front of one rocker panel to the back of the other, with subframe connectors running straight through those legs or welded or bolted to the legs of the "X" would seem like a great idea, but I'm not sure about an X-brace that just bolts to the front and rear subframes. There are lots of ways to improve rigidity, but what's the best design? I'm sure the TCP X-brace helps, but there are a few issues to consider: (by the way, I've spent more money than I care to admit on TCP suspension, Wilwood brakes, KRC PS pumps, etc., so I'm definitely a fan of some of your products)
1. The OP has a convertible where much of the strength is in the rockers, and the TCP X-brace completely ignores the rockers like almost all of the subframe connectors out there. I don't think it's a bad thing to take some of the work away from the rockers, but I think it would be much better if the main structural elements were tied together.
2. An X-brace that goes below the convertible floorpan lowers ground clearance - convertibles are already challenged on this.
3. You raise a really good point about an X-brace in general limiting the travel of each of the corners, but how much does the TCP brace do this? It's not a pure "X" - is there any twist along those center tubes?
4. I think if I were to start with a clean sheet of paper, I'd build a huge "X" with boxed-in legs molded to the floor pan so they could be skip welded at a few points to the floor pan and with the subframe connectors running through the legs of the "X" to give some longitudinal rigidity and tie the front to the rear. You'd have to cut into the convertible subfloor, fit it in, and weld it to the subfloor. Driveshaft and exhaust clearance would be a problem. It would tie to the rockers at the front and back, and in the center with the convertible subfloor that runs from rocker to rocker. It would be heavy.
Convertibles are tough . . . there's no simple or easy way to reinforce them well . . .