Alright, MAJOR development here, we put a prometheus MS110 in breakdowns FAL, shot 280fps. OK could just be a mislabeled spring right? Well I put the stock spring into someone's gun and its shooting 370fps now. And we put a pre-tested 388fps spring into the FAL and it still shot 280fps. So theres a VERY consistent 100fps leak somewhere because its only shooting +-5fps.
Best bet so far is the nozzle a bit loose, it may not be hitting the hopup rubber properly, but that usually means a greater variance in fps, so at this point I really don't know.