we ordered some lightened bumpers for Domino the other day. They are not very expensive, roughly $75 each. What is the difference? I would guess maybe 20 lbs less weight each. Essentially the lightened bumpers only provide enough support to having something to attach the bumper covers to. Apparently they provide enough crash protection...if you lived in Japan since they are roughly the same as the JDM GC bumper frames. The pictures explain more clearly what they are about than I can in words.
Here are the part numbers if you want to do this to your USDM GC Impreza RS:
57760FA680 - Lightweight Bumper Beam Front 93-98 Impreza
57760FA400 - Lightweight Bumper Beam Front 99-01 Impreza
57760FA192 - Lightweight Bumper Beam Rear Impreza
Lightened front bumper frame on top, stock on bottom.
Rear lightened bumper frame on top, front on bottom.