I have a two recommendations for you. First one, are you absolutely set on using co2? Propane/Green gas mags are just as cheap, and are much better for your gun. CO2 is such a violent gas, it will slowly beat the shit out of your gun. Propane will most likely give your gun a longer life expectancy. Second suggestion, is to buy a RA-tech steel trigger box. WE m4s have a steel buffer lock, but aluminum trigger components. That aluminum is known to break on WE guns, and that's what happened to my WE m4, and WE scar. I would worry about performance once you know for sure that your rifle will last more that a few months. Really hope I could help you, all of this stuff unfortunately comes from MY past experiences.
My WE 416 (same as M4 internally) shot around 490-520 on my CO2 mag, and 460-480 with the green gas mag through a 14.5" barrel. The temperature was a little different when the mags were fired but it gives you a rough idea. Long story short you will need an NPAS regardless of what mag you use. It's also worth noting that the gas capacity of those 12g bulbs is pretty close to 30rds. When using my CO2 mag I sometimes couldn't get through all 30 rounds if I shot too quickly. I currently have all V2 green gas mags and have no problem locking the bolt in 70+ degree weather.
I have heard the stock WE bucking is pretty terrible. Both of my WE 416's came with upgraded buckings so I have no personal experience with it. The good news is the open bolt system uses VSR10 buckings which are super common. I have always had good luck with 9Ball purple buckings. I have seen tight bore barrels but I have no experience with them as I'm still running the stock (polished) 6.05 barrel with great accuracy. Taking the hop up apart and shimming everything up is important as there is a bit of play. Outside of that I haven't come across anything else worth doing as the VSR rubber is a pretty damn good design.
As far as upgrade companies go I have read a bunch of reviews stating that RA-Tech has been using softer steel lately. I have a few of their parts and they seem fine to me but I haven't really tried to test their hardness. I have their full firing pin set, disconnector, and bolt catch. The firing pin was a bit too thick so I had to mill it down and the frame was a tad too wide. My disconnector guide was about 1mm too high and required a full new contour cut into it. It is also pretty common for the RA-Tech hammer to be taller than needed which causes problems with the bolt. I came across this review which shows CWI trigger parts vs RA-Tech. I'm planning to go with CWI. Either way you will be custom fitting parts to your rifle. GBB parts hardly ever just drop in and work. Chances are you will have to go over seas for most all of your upgrades. Get used to it, the fun stuff is over there.
To be honest you should learn to do the upgrades yourself. Being a GBBR it will require more maintenance than an AEG. You should be familiar enough with it to take it apart and put it back together so you can clean and lube the areas that will require it. If you let your bolt run dry or run the same dirty sandy lube for too long it can cause major problems. Same with the trigger group. If you do find a tech willing to do the work make sure they quote you for the time it might take to machine the parts and make sure they aren't going to hack them up and hammer them in. I spent a few days carefully fitting my firing pin and disconnector as neither was a drop in fit. If I was still in the tech business I would be charging a pretty penny for all that time.
Other things you should do:
- Check the buffer detent to bolt clearance and adjust if necessary (it's commonly too high) - Round and polish the full auto sear tip. - Clean and polish the trigger sears and check them all for burrs or sharp edges. - Tighten all screws and blue Loctite the ones that need it. - Make sure your buffer tube is tight
Co2 proof a gun? I use a WE 416, and I run both green gas and co2 mags. The trigger? Don't replace it with anything until its broke, and I say this from experience. I run co2 all the time and have over 10,000 rounds on the stock trigger box. The thing you need to be aware of with using co2 is your loading nozzle. Spend the cash and get the RATECH aluminum loading nozzle. Your gas consumption will go up, but you can go from 300 fps to 600fps, and you won't have to worry about breaking your loading nozzle during cold temps. Do get a 368mm angry gun 6.03 tightbore barrel. Airsoft global usually has them in stock. 6.03 will be fine, 6.01 will just give you a tighter grouping but you will lose range. You also want to buy the reaps A+ hop up bucking. Evike usually has them in stock. At first your performance with all this is going to be shit. It literally takes about 1000 rounds through the gun before the nozzle, and hop up get broken in and settle correctly. So you're going to get some great range time to get familiar with the way the gun performs. You can always p.m. With questions, and I hope this helps a bit.