If the ASG is a clone of the A&K (which most of 'em seem to be) then it's cheaper than most to get up to skirmishable standards, but there are some essentials that you need before even bothering, IMO, even though it should reach about 440FPS OOTB. The stock hop unit is such a ball ache - it needs a hex key into the top to adjust, which means the top cover has to be off.
The advanced one from airsoftpro.cz is easy to fit and works like a G36 type but a bit more precise, so you can adjust it simply by drawing back the bolt catch (although that is also cocking it, so if you let it return forward after cocking, then "open the chamber" to adjust the hop, you'll have put a 2nd BB into the hop unit - you need to cock then hold the bolt to adjust the hop, then let the bolt catch go).
Also the bolt catch is quite small to pull an M150 spring back - with one, maybe one & a half fingers, so you need to buy the extended handle -
here it is with a good pile of bits of varying necessity. You can get most of it from Daz on ASPUK too, but not as cheaply. Even so it's a fair drag to cock...
I put in a Madbull Ultimate 590 x 6.01mm TBB which upped the FPS to around 540, which meant I could drop the spring to an Element M135 to come in at around 492 with 0.2g BB's - ppl say you can fit a 650mm barrel in them, but that puts the inner barrel well into the flash hider. visible through the slots, besides doing so may well make it difficult to acheive top FPS without going over 500, so you'd have to drop to a M130 and lose 10-15FPS, down to maybe 385... anyhoo, it's a lot easier to use like this.
Edit: in reply to Remus above, doh!