Interesting findings for the Gamo Center which does not suprise me. I used one heavily during the early 80s and found it very accurate.
There was some argument back then that rear moving pistons for Webley pistols were a better idea but time has proven this to be in error.
While it appears to make sense to have a rear moving piston striking into the hand, they forgot about the 2 way recoil effect, where the secondary recoil moving forward (in the case of the Webley design) is more difficult to control than the secondary recoil of forward moving pistons. You will find that the Original 5, Gamo Center etc are more accurate than the Webley, HW45 designs offhand therefore.
Lightly sprung at approx 3ftlbs the Gamo didnt suffer from much in the way of recoil/vibration niether.