With long term tendon pull problems in both my elbows my heavy Hatzan has sat in it's case for about 4 months now. Wanting to still shoot something I brought a Beeman P17. Think I got lucky with mine too as it's currently averaging about 3.1 ft/lbs according to my Combro mk4 crono after 100 shots. Just 7fps difference over 10 shots using H&N Barracuda Hunter Extreme (used in my Hatzan). 'Used to be Eley' Wasps are a looser fit in the barrel and gave a 16 fps difference over 10 shots.
Just put a pistol scope on it today. It's still a stock pistol though I have wipped out the piston to check for any burrs on the air intake but found none. Apart from fixing seal / firing problems I've not found any UK custom jobs showing up here like a couple of forums across the pond. Not joined them, just been researching the P17 before spending my money. Let us know if you've been tinkering with yours, I like to know what tweaks have been tried and what parts you used if required.
Cheers!