10 joules is roughly 7ftlbs.
Flat top piston and valve is a good start and will increase power.
An aftermarket breech and extended bolt is a must for me. The crosman steel breech and gmac's hollow extended probe is a good combination. Replace the metal transfer port with some poly tube the same diameter prevents lost air. A smoother hammer spring makes shooting nicer as well.

I've been tinkering with my crosman pumper carbine the past couple of days and am getting 10.5ftlbs with 13pumps. But mine has a longer barrel and i've modded the valve insides. I reckon with the stock barrel and all the above mods you could get around the 10 joules mark with 8-10pumps.