Frame, chassis, its semantics and they are used interchangeably. The Gen 2 RT-10 & GTS share the same frame and obviously if you twisted the bare frame they would have the same torsional stiffness. I've never seen a number stated for the GTS' stiffness increase over the RT-10 other than Popular Mechanics who said 12% which I think is a bit vague and left to interpretation.

Although screwed and glues on, the addition of the GTS' roof does increase rigidity. You would be surprised how much most T-top and convertible cars are affected by the installation or removal of their hard tops. Most people probably cant feel a difference or can drive their car proficiently enough on track to feel a difference, but if you took a complete car to a K&C rig, the GTS would be stiffer than the RT-10. I doubt it would be huge and don't think it would even be 25% stiffer, but it would have more torsional rigidity. There are some good quotes in that link in post #7:

"2003 roadster was 31 percent stiffer than the old roadster, and even more rigid than the old GTS coupe"

This reads like the GTS is stiffer than the RT-10. Also while its quite a bit different and apples : oranges, the Gen 3 coupe is said to be 30% stiffer than the SRT10.