Hi El Garo, I have messed quite a lot with my mk1 .177 "S" and it now shoots in the high 11's with superdomes. The spring in it is a titan no.1 with loads of preload but it is nice to shoot. What I did discover was that if the pellet was low down in the tap it knocked ft lbs of the muzzle energy. That's why the domes give the highest power as they have a large diameter skirt which holds them high in the tap. However, it doesn't group very well, giving its best with hobbies which are only slightly less m.e. than the s'domes. I am not sure if the spring is the cause being too strong and may one day fit a std airsporter spring. One definite disadvantage in achieving accuracy is the trigger. Not such a problem with open sights but add a scope and the short comings become more obvious IMO.
I have a mk2 .22 s which refused to go over 10 ftlbs, I am currently waiting for a new custom loading tap with o rings fitted which will hopefully improve efficiency.