makes sense to me -- less dense material = less vibration damping. also as i recall, the stock pulley had a piece of rubber sandwiched into the middle, probably for that exact purpose. silly me.
i just don't get why there wasn't a problem until about 6 months after they were installed.
whatevarrr...i'll see if my friend still has the EJ18 he pulled out of his brighton laying around the shop, and just trade him for the OEM ones.