I haven't chimed in here because I think my own thoughts have already been captured by others. I don't see the "arms race" as a problem...more of a natural progression. And because I've picked up a spring gun on the odd occasion over the last few years and shot match high score or beaten 95% of the PCP shooters in attendance on that day, I can pretty confidently say that I could do the same with a relatively inexpensive PCP gun...but maybe that's an experiment to run another time. In general, if folks spent more time purposfully practicing their scores would likely go up. Not much more to it than that in my experience.
To answer your question, before Bill made the switch to Thomas back in 2018 (I think), I would say it was pretty regular that he was winning Hunter PCP at local matches and most GPs he attended. Not like he won everywhere all the time, but you get the idea. So if we took Hunter PCP as our example, and then factored in matches before and after as well....around 3-5% is probably appropriate. Guys like Bill and Philip are the outliers, no doubt. But while the equipment they used and won with made them outliers, the actual reason they were at the top is far more important and something not unique to either of them...they PRACTICE!