Piston head should increase the compression significantly. Of course if you want it even better still, replace the whole shebang, cylinder, cylinder head, piston, piston head... Although that is generally the same story with any AEG.

is it possible to do the same upgrade on a G&G MP5 A5 Blowback? I would really like to upgrade the whole kit.

I have not touched the blowback ones...

It's a gimmick I'm not willing to deal with. There's no reason that these things should have blow back in the first place.