It's like anything else, gas guns (propane, co2 or otherwise) need to respect the field limits. It's not like AEGs or spring -powered BAs can't shoot above 400 fps, too.
Co2 doesn't necessarily shoot hotter than propane: if I take my WE M4 mags as an example, in side by side conditions, the propane ones usually shoot hotter in most conditions (the interior of the mag is entirely different as well).
So the question is, would you let a hot AEG play? Or a hot GBBR? Or would you oblige the player to install a weaker spring or NPAS, etc ("downgrade or go home")?
If the handguns/SMGs don't conform to whatever rules you have for FPS, then why are you accepting them? And if you don't have rules then you probably should.