I plan to do something similar to what you are doing. I have ordered two plastic airbrake piston heads from Lee's Precision Engineering (
http://www.leesprecision.co.uk/). They are not very expensive even when converting to CAD$, shipping was incredibly cheap and fast (5 days or so).
Now, I didn't have time to install one in my M4 yet, but I did install one in my A&K spring SVD, and although I cannot tell you anything about durability and timing in full auto AEG, in my SVD it made an incredible difference in sound level. It went from louder than an AEG to about as loud as my TM VSR10 G-Spec, without using a sorbo pad. The FPS was exactly the same as with the Prometheus piston head I had installed in there, which was quite a surprise. I had expected some loss of FPS, but it looks like the SVD cylinder has more than enough air volume to handle the airbrake.
I will be testing one in my M4 in the next few weeks. My main worry is that it is going to have pre-engagement of the piston and gears because the airbrake slows down the piston, but I plan on using torque gears and a torque motor to reduce ROF and prevent this.