I've seen speeds in the 360s with the HCA arrows years ago (80 pound bow, roughly 3 grains per pound), so I don't see why 370 would be out of the question. IMO, it's a disaster waiting to happen, but thats a different thread.

I'd say a modern bow like the Monster or 82nd would shoot well into the 370s and higher with those arrows.

And I think you are confusing GPI, with GPP.