I think single piston springer tuning has peaked.
Tightening tolerances and using the best materials for the job is the best and obvious way to register a marked improvement over factory standard jobs.
It's when the dynamics, tp, stroke, piston weight etc started to be fettled that it moved up a couple of notches, particularly when one throws a maximum power that must not be exceeded into the mix.
Perhaps single piston springers were the go to gun since they were more " expendable" than say a Park, Whiscombe or Giss Diana.
In my day, everyone was fettling Minis and MK1 Escorts because they were affordable and easily fixed if things did not work out, which was not the case with car's that were already fettled from new.
I doubt if the twin piston airguns which are still around can be dynamically improved further except perhaps take advantage of today's CNC machine tools to reduce slop and production variables.