I am not a "tuner" and only tinker. The only time my rifles have been chrono-ed is when a friend brought his. Having read a few things on here about tuning, it seems that using a Meteor sized O ring helps with power and bedding in---although a friend had a Mercury with a very old O ring that was "hot" until he cut the mainspring.
I'd try to push the the piston and piston head, without the O ring into the cylinder and see if there are any tight spots. If there are not, then I'd try the Meteor sized O ring and the new mainspring and maybe the old mainspring if it is not too bad, after that.
If the tap is ok and there are no really tight spots in the barrel, I would be thinking is the end of the cylinder leaking like hapens on Weihrauchs, although I've never heard of it on an Airsporter but had it on Webley Hawk.
I bought a rough Mk5 off here a good few years ago and that had the siezed nylon pisotn head. I just cleaned it up, fitted a new O ring and found an old mainspring that I liked the "feel" of for cocking and plinking. That was doing just over 8.5ft lb and was lovely for plinking.
Good luck with it.