Personally I have a CYMA AK 105 and after some aftermarket upgrades (that cost me $100) plus a tightbore 6.03. My gun is shooting as straight as my friend's G&P M4.
This gun is ultra reliable and tbh I wish I did not had done all those upgrades from the go because I know an other guy who has one with almost stock internals and never failed him.
I don't know about all the CYMA line though, but CYMA AK's IMO are very good for the price.
Also FYI with a 455mm tightbore barrel installed and a SP90 modify spring, I am having a velocity of 340-345 FPS. So, if you want to do only the minimal in terms of upgrades (since you will have ot open the gearbox anyway to install the new spring, might as well clean-up,reshim and change the bushings at the same time)
-S90 Modify spring
-6 mm metal bushing to replace the nylon/ruber one.
You can find all this on airsoftparts.ca