I’m running reliably at 350 FPS on green gas (Red gas if it’s very cold) with nothing more than a tight bore barrel and an NPAS. I could probably run higher, as there is still room to adjust the NPAS further, but I would need to lock it to semi before I tried and I don’t want to semi lock this gun.
My next Airsoft project will be an MWS mk12 DMR build and I think I can easily get to 450 on mostly stock internals.
One comment, on HPA it’s not necessary the FPS that causes problems, the potential for insane rates of fire can put a lot of stress on the internal parts. Keep the ROF realistic/sensible and at 350 FPS the stock parts should be fine.