Ok so the single 6-rib LCP for the '15 WRX won't fit on our cars, but... The more common 2 belt pulley will. "But Dave! We only have one belt, and it's got 6 ribs!"
So? I've been running a 5 rib belt (a NAPA 25050650) for the past few months as an experiment. I haven't had an issue yet, regardless of alternator load, hitting the steering lock, etc. So... I put this baby on with my 5 rib belt. The 5 ribs on the pulley line up with the 5 forward-most ribs on the accessories. Went from 5lbs 11oz to 1lb 4oz (2590g to 565g).
http://i.imgur.com/kxa23GL.jpg