I have a 99s in .177 which was rather snappy and twangy at first, I have fitted a wonkey donkey service kit along with de bur, polished the piston and regrease, it's now lovely to shoot. I have no doubt mine could be made a lot better if I sent it to Dave but I'm happy with it as it is now. Just got to find time to shoot it more.
Paul short is another very good tuner.
There is a lot of shooters who have not been that happy with the SFS IMP , mine has been ok and many other too
I know a few who have installed Wonky Donkey kit in the imp and they say it's better than before , what i do like about the imp is the trigger