Hey mate, no need to extend the barrel, it should be fine assuming everything is "straight" lol, & you might actually benefit from some sound suppression if the silencer is foam filled.
As for internals, don't do anything YET, there's a number of different editions of that rifle, with differing FPS, often dependent on where it's bought from, but luckily for you it has a quick change spring system so you can easily experiment with the FPS without upsetting anything else.
Cyma's can be very good straight from the box, & I'm a big believer in "if it ain't broke don't fix it", in its current guise you may find it fills all your needs, only thing you need to be careful of is your sites dmr rules & also UK law as I'm not sure if your model has the ability to shoot full auto ?, if it does you may have to modify it for semi only .
Good luck?