I have used the JG M4 GBB, out of the box 470 FPS with green gas and 0.2g bbs. Just improve the seals, fix the hop-up na dili siya mu hop pag ayo and get the R22(red gas). Mine clocked at 510 FPS at 0.2g bbs. Contrary to GBB having low maintainance than the AEG, this is not really the case esp for the China made ones. The china made ones tend to become brittle over repeated use since the cold release of the gases coupled with impact from the moving parts like the hammer, cocking lever and the bolt. Pero if palit ka adtong mga Inokatsu, lig-on jud, it should be, considering that price is around 20k php. The o-rings also brittle over time since rubber o-rings were designed to withstand heat but not cold. Another issue is the magazine, currently there is no stock all over the phls for china made gbb mags which cost 1200php, if you have the moolah then you can buy the high end gbb mags, i dont know the price but if the china ones are at 1200 each, you should get the picture. Also these mags are almost half a kilo in weight, and houses 40 rounds only. You would need 10 mags to have 400 rounds which is already 5 kilos on your vest. Lastly, the Gas, you may consume 1 canister each gameday or even more. Bnew costs 400 per canister and refills(if you find a reliable refilling station) are usually at 250 per canister. So you can do the math.
Basically, my review of the Gbbrs, as of the moment, is good only for tactical shooting or plinking.