Don't know exactly how it would effect your results, but I read a little about transonic something or another. Basically, it only takes 16" to burn the powder in a 22. If shooting hypersonic ammo in a 16.5", somewhere down range it will go transonic and disrupt the accuracy. The 20" would have a few inches to slow the bullet down to subsonic before leaving the barrel....in theory. That said, if I was shooting a rifle with a match chamber, such as the mtr...I would shoot match grade ammo in it....and it's subsonic.
I understand that you haven't had good luck with eley...but that was never the case in any of the 20 or so CZs I've owned. SK was also good ammo...I just got better results with the subsonic stuff. And I'm not talking about the 600 fps stuff. It's only a little slower than the regular stuff...to avoid that transonic transition.