With the "Original/Diana" factory 33 coil Spring with 276 gram piston, chrono results were :-
Hobbies = 580fps
Superdomes = 500fps
So I stripped the gun again and weighted the piston back up to 305 grams and put the 28 coil Spring back in but added 4mm of extra preload to see if it would lift the power a tad, results :-
Hobbies = 610fps
Superdomes = 590fps
So the tune I've now chosen to go with is the 270gram piston with 28 coil HW95 Spring with 16mm total preload that gave 11.6ftlbs with Hobbies.
11.6ftlbs will do me for a bit of paper punching in the garden and dealing with the occasional rat.
All the best Mick