The WRX already has a pretty light flywheel. The problem you get with a lighter flywheel is less torque and more driveline lash. Less torque makes the car more unhappy in stop and go traffic. Driveline lash is very common on AWD cars since they are so grippy. The lash can cause hesitation and bucking if the flywheel doesn't have enough mass to act as a damper. A different BOV or turbo controller is the way to go, although I have little problem keeping my car on boost, and it's pretty near stock.