The information in this thread does not match the other thread
http://www.airsoftcanada.com/showthread.php?t=158580
Is it an M120 or an M130 spring?
400+fps with 0.25 ~ 447fps with 0.2g. You're not going to hit that with either spring. 360-370 with 0.25 is about right for an M120 spring.
Since you have poor fps and poor efficiency, your issues could vary wildly: anything from poor shimming, poor wire quality, poor electrical connections, poor choice of motor, low-quality battery, poor air seal on the piston head/cylinder head/nozzle/hop-up rubber/hop-up unit, improper selection of cylinder size.
A SCAR-specific observation is that the SCAR-H has a retardedly long nozzle which is proprietary, and last time I checked it didn't have an o-ring on it so that might be an air seal problem. Also, with a long nozzle you may run into air seal issues with fully-loaded mags causing ammo to push upwards on the nozzle, creating an air leak.
Contact myself or a reputable airsoft gunsmith to fix your gun.