Systema, G&P, Matrix are all good.
It makes it so the gears move more smoothly so it causes less wear and tear and can possibly also up the ROF a tiny bit.
Check out the guide for shimming on 858Airsoft (the forum shut down for some reason but I believe the guide should still be there. Just search "V2 gear box shimming" and it should be one of the first links on the internet.)
You'll also want to correct the AOE and re-grease it. Google how to do those.