The frame rate won't really be any different, there'll just be a little bit of extra delay until you can actually see the frame that was just rendered, and maybe some barely detectable stutters. Keep in mind we're talking 4K120 or 8K here, the GPU in the Steam Machine is nowhere near fast enough for either of these things. It's likely that there's always some mode with a higher fixed frame rate than what the GPU can calculate, even on Full HD you can probably just do fixed p120 or p144.

VRR is all about smoothness/microstutters, not higher frame rate/performance.