frankly i don't see why anyone would feel the need for anything higher than the ROF on a kwa with a 9.6. and the gun has EXCELLENT durability with that fairly high ROF and 400fps+ capability. all the gun needs is an inner barrel. only compatibility issue is the hopup, and you don't want to replace that. at worst the lower hopup cracks or something and you have to buy the part from kwa for a reasonable price.
no experience with the others, though there are things i like about both. would like to try the 2 piece ICS gearbox at some point, but for a solid field gun out of the box i'd say kwa. G&P, G&G, and other taiwanese manufacturers have decent guns but they clock a decent bit lower on the chrono out of the box. VFC's are more up in KWA territory, and they're some solid guns. gearbox doesn't seem quite as advanced as the KWA, though.