in my opinion for indoor and outdoor rifles you have 3 options for aegs. aries ics and systema ptw, they all allow for "quick" fps change.
there is also a adjustable nozzle you can install into your mechbox that allows for quick fps change. someone else might have more info on these as i've never seen one in use.
also there are gbbrs which can let you change fps quickly. this can be done by switching the type of gas you use or npas which allows for flow change. i'm not a gbbr guy but im sure there are more ways to change fps in a gbbr.