Daft question , but are you filling them correctly ? mag upside down . All of my WE stanags will happily do 2 loads of BBs with one gas fill, actually it's more like 2.5 loads but I rarely need to use that many. Mine are used in my L119's but also in my L85 which is the same spec as yours , RAtech NPAS and washers on the recoil rods.
How free is your bolt ? if it's dragging that will use more gas , have you checked the O ring on the back of the nozzle ? again if it's shot it'll be using more gas .
Do you knpw anyone else with a WE that takes stanags so you can compare perfomance in another gun ?