The 2240 makes a fine carbine, the one I have with a long .177 barrel is a nice shooting light gun, the 2240 in stock form is loud because it's stock barrel is so short so it's not efficiently using it's CO2, if memory serves an 18" barrel is the best balance before diminishing returns... I grabbed the file from work, it's not as complete as I thought, I don't remember what I did with prices and the rest of the part numbers I've used over the years... I'll throw them up here after a bit...
Most of the tuning on airguns is on the triggers and smoothing out the actions, almost everything has a 10lb trigger stock or even worse, that's likely the main mods... On the Crosman pistols the easy way of making the trigger smoother is removing a grip panel (single screw), pulling the spring you see with the gun not cocked, over compress the spring (can be done with your fingers or pliers) to the point it has about 1/8" of preload when put back in, but before you put it back in take a spent .22lr shell casing and put it on top of the spring before you put it back in. That'll leave you with a smoother trigger that's about 2lb pull...
Flat top piston just reduces the volume of the piston assembly... stock they have a rubber cup that doesn't actually fit the face of the valve so there's some dead space, the gains aren't too great until you exceed 5 pumps but there's crazy people that pump those things like 50 times to get every ounce of power there is... I have a Powermaster 66 that I put a flat top piston in that at 15 pumps it gives one shot at the same velocity of 10 pumps and a second shot at the equivalent of 5 pumps which is kinda neat...