Not sure why you are saying they're bottlenecked. Hardware not being at 100% utilisation isn't a bad thing, it means that they A) don't have to run as hot which A2) means less power draw but also B) CPUs are getting so powerful that they can compute enough instructions per cycle that they can run the games just fine in addition to their ordinary tasks.
What's the point of this? Getting 350FPS at 1080p instead of 250FPS? Pointless except maybe for those edge cases you mentioned, who are also going to be more interested in 3k or 4k resoutions. To me, this is like saying 'why doesn't every car run at fifth gear all the time? Why not give every engine the torque to pull a boat?