Just finished put a JM tune kit and new piston seal in my 34. It really is smother now. Have only shot 5 times it started to rain. But all 5 shots were at 10 yards and it put 5 crow-magnums through a 3/4 inch pine board. Trying to get the cups for the front screws from Mac1 and then am going to redo the stock.
From my experience, these kits are designed to produce a smoother cocking and firing behavior, not necessarily to increase power. I installed the GSI kit and Apex seal in my 34, and it did indeed become a very smooth rifle. I never chroied it before the kit, but after, it shoots CPLs right around 880 fps. Its the most powerful spring rifle I own, and also the most powerful I need. Most of my other guns are sub-700 fps. Even so, the "tuned" 34 fits nicely in the crowd of low to mid powered spring guns I prefer to shoot.
