The power list is long and expensive. If I was all about the quiet power I would get a 10/22 with a muffler and pay the $200 tax.Who out there makes the hardest hitting air rifle in .177 or .22 cal. At a reasonable price ofcourse. It doesnt matter if its a pump, break open or psp, co2.
I have an old RWS 45 that will send Crosman Premier 7.9gn pellets super-sonic once a blue moon.The speed of sound is the enemy of all bullets. It screws with accuracy with the best of streamlined boattailed bullets. Wasp-waisted, blunt nosed pellets don't stand a chance.
So if you've got a magnum-powered .177 airgun, you probably want to shoot a heavy pellet to keep you far, far away from Jimmy Buffet's evil brother, Mach. If your springer produces 20# ME, you'll be going supersonic (@STP) with anything lighter than 7.7 grains. If you want to give Herr Mach a respectful margin, a 9 grain pellet @20 fpe produces about 1000 fps. The same rifle would push your 10.2 grain Diabolos to ~940 fps. That's enough zing to put the smack-down on most birds and small varmints, plus there's no chance of interference from Mach buffet.
So the short answer is yes, if you've got a sufficiently powerful airgun, it's essential you shoot heavy enough a pellet that it'll remain subsonic. And that particular pellet seems to have very good reviews.
All this ignores the question of whether kinetic energy or momentum is more important to killing power, which is a whole nuther flame war all its own. :whistling: