the power adjuster is only a mean to fine tune the power you already have. it will preload your hammer spring but in my opinion that is not necessary. Free almost power is available with a bit of porting of the air port on the top of the valve. Use a dremel and make sure that you leave no burrs. then go to sears (hardaware store ) with the spring that sits behind the hammer. go to aisle with springs and find a replacement with stronger tension (be careful, to much tension will waste co2...). Another little trick i use is if i want to use the original spring, i get a 1/4 inch bearing and put it behind the spring. It preloads the spring for you. obviously, you may use two (front and back) but you may not be able to cock the gun afterwards... so try with one first and see if you like it. You may also use a plastic fish tank hose as a transfer port as long as it has a larger I.D. than the port. If you cut it a little longer than the transfer port (tiny bit) it creates a better seal). so far that is free or almost free.
The barrel lenght i agree that you might see about 5 fps per inch, so if you got it use it.. then clean the bore until it sparlkles (it might just be me but is good for 5 to 10 fps increase as the pellet finds less resistance on a polished bore, i have tried it in 3 guns and the gains are there, as small as it is... and it cost you nothing.)
So far you should see between 550 and 600 fps with crosman premiers. And with those mild tuning tips, i still get 27 to 28 consistent shots at 65 degress. when the weather gets hot I have seen 630 fps. Also see if your gun likes RWS hobbies... at 11.9 gr mine spits them out at 655 fps, little ragged hole at 20 yards, about 1 inch at 40. But they are devastating for hunting birds.