I was thinking about this myself since I just picked up this exhaust used and do some amateur motorsport stuff. I will keep you posted. Was thinking about building my own brace after trying out the Thermal one. It just needs to incorporate a bolt hole for the hanger that they incorporate into the crossmember. While the thermal crossmember design isn't ideal for strength, it is very thick so it may be sufficient. I also think this area of the chassis is strong to begin with. The fiesta R2 doesn't even use a brace here (granted it is seem welded).
View attachment 26512
That poor little rally car.
It's obviously in tarmac spec, given those tires, and the lack of a skid plate (unless one was installed anyway, and he tore it off in that off/roll).
Again, the whole full FIA spec cage is tied into the unibody
everywhere (on TOP of all of the seam/stitch welding you've mentioned), so that negates the need of that center 'brace', in a rally application.
BTW; that pic above is a good depiction of the M-Sport kevlar under-body protection.