I'm not so sure an FX Impact is an airgun you "mess" with. Once you choose your fodder, buy the best liner for that fodder, properly tune it as Ernest Rowe suggests, there isn't much more to do but shoot it!
Well, so far with mine, I have installed a hybrid PP kit, modified the probe, changed to a dual ports. Installed the full PP kit, picked up four liners, cast slugs and pellets for the specific liners, tuning to the specific liners to many different projectiles, extended the rail, cheek riser, different cheek rest, bag rider, sling mount, cleaned out the Shogun, replaced some O-rings, tweaked the trigger and some other tweaks I'll leave out.. Basically, I want to know what this gun can really do. It's new to me and I don't mine making it sing by messing with it. It's my gun now and it gets my full treatment. I hunt, and hunt in .25 cal and .30 cal depending what I am after and then I like to plink with less fury in the backyard. This gun does it all quiet well. I'm not one to buy an airgun and be afraid to turn a screw on it. What I don't know I'll learn as I go. Yes, at some point I'll settle down on a couple of documented tunes in the different calibers after the fun of tweaking runs its course and I am happy with those. Then it'll be time for some other airgun project. I do leave it alone when I have a hunting trip coming up soon.