Not a member there, but it's very plausibly not entirely bullshit. PCCs shoot 200-250 fps faster with the same bullets. At the outlier, 45acp +p 165gr out of 16" barrel is like 1630fps. A 30-30 is 170gr with 2200 fps.
There are bullet traps that are rated for 308@3kfps: chances are, you don't have one as they are expensive. If it's a rubber trap, using pcc would make it ablate/break down faster - probably unacceptably fast. If it's steel, then it may ablate if you don't limit to non-jacketed bullets only. Also, lead dust is an issue: more pewpew, higher blood lead levels without better ventilation. Oh, and sound dampening is a thing. I know other people are ginger about shooting their space guns, and even my 45 is a little loud - why make everybody else double-up on ear pro?
So, do you want to add a dozen arcane rules that, chances are, you can't reasonably enforce, and people will blow off, and might not allow you to shoot what you want (i.e. limited to 9mm non-jacketed, subsonic only, no comps since it might...I don't know, set the ceiling soundproofing on fire or something, also you have to carry extra earplugs for everybody if they want, etc, etc)?
Otherwise, find out how much it would be to bring the backstop, ventilation and sound dampening up to snuff for PCC, and pony up the difference, or take up a collection amongst the other PCC guys, sell baked goods, hold a fundraiser PCC competition prone-only outdoor shoot in the rain to underscore the need for an indoor range, etc.