After reading the VTS for the C6 Corvette, I realize that they're using an old Dick Greenwood trick.
The Corvettes, being a full frame car from the factory, are tying the cage directly to the framerails and using a one-piece aftermarket composite roof/lexan windshield unit from ACP, the same folks who produce ALL of the legal bodywork (and the sole legal body modifications) for the C6 Corvette.
This is actually quite an old thing to do with glassback Corvettes, dating back to IMSA Camel GT Group 5 competition, as well as Trans Am, with Dick Greenwood's Corvettes. Even though they wear silhouette bodies and the roof is no longer structural, they're still running a stock frame. They're what us short track guys often refer to as "clip cars" because they use factory front and rear ends.
What's a little odd, honestly, is that because of the Corvette's perimeter frame engineering - instead of unibody - the SCCA is allowing C5 teams to update their C5 chassis into C6 cars by attaching the aftermarket body panels, the allowed bracing, and re-engineering the rear suspension with bolt-on C6 parts (namely the cradle and upper control arms).
I agree with you, that brand of construction - which the Vipers share, as they also are a factory perimeter frame car with a tie-together cage - gives those cars a tremendous advantage over the unibody Skyline when it comes to weight savings. However, to call them "tube frame" is a bit of a stretch as a tube frame's purpose has nothing to do with weight savings, but instead is intended to relocate or replace suspension components.
A true tube frame car, in fact, abandons stock geometry and components altogether. Trans Am cars, for instance, share 0% of their chassis with their factory namesakes, using sophisticated four or five link solid axle rear suspensions and minimal travel tubular a-arm fronts with practically no roll.
Not to be disagreable, Sean, but coming from a background in ISMA and MSA Supermodifieds, if there's one thing I know it's a tube frame, and neither the Vipers nor the Corvettes in Speed World Challenge are true tube frame cars.
It kind of makes me wish the Skyline GTR was a full-frame car instead of having a godawful unibody. If it were a full-frame car, it'd be easy peasy to slice the pounds out of its hide, get 'em down low in the chassis, and have you a setup like those C5/6 hybrids and the Viper Competition Coupes.
I'll be doing all I can to be at the Mid-Ohio round in full support.