What exactly does 'proven' mean?
It's not like 140 grains at 3050fps is a total mystery. I can say this with 100% certainty and I wasn't even there --- the first animal ever properly hit with a PRC died in a very, very predictable manner. It didn't 'prove' anything, nor will the next 100,000+ animals that are killed with it.
If I've got 800HP, is 850HP worth chasing?
Again, you use the 270 as the starting point, but that logic goes both ways. If I have a PRC why would I buy a 270? There's no need to 'chase' anything, it's no trick to get a PRC to hit its published velocities.