I know a good chunk of the math behind how a pcp operates from valve strike to pellet hitting target, thats a LOT of information to gather / educate yourself on over time. Albeit, I still have tons to learn.. The knowledge I've obtained applies to all rifles regardless of brand...the biggest change between brand that may effect my calculation that many people won't know is volume from valve seat to base of pellet, or barrel tightness / choke...that can vary widely with some guns but even then that variable is marginal in the grand scheme of variables, which is caliber, barrel length, pressure, port size, plenum size (I pretty much always say provided you have enough plenum when regarding it), and pellet weight. Its not too complicated, believe me. The weight of the air from the discharge is also of importance. I can go on and on but the primary ones will always give similar outputs all being reasonably predictable.
I don't suggest for anyone to take a drill to their rifle regardless if its 200$ or 2000$...anyone should know the risks when attempting modifications that require removal of material. I may however advise of a potential configuration or port size that will work for them that they can request from a tuner OR attempt on their own...we're all adults and can assume responsibility for our actions I hope
A 500mm barrel should perform near equally in an fx, crossman, kral, daystate, cricket, ect...provided everything else is equal or very close... pressure, plenum, caliber, pellet weight, port size...Now accuracy from said barrel, form factor of said gun, air capacity / volume of HPA changing shot counts is what really separates brand names at the end of the day. Anyone can spend a little coin and toss a 600-700" barrel on any brand .25" and blast 33.95's out at 1k fps provided they do proper modification at the end of the day.