Funny, I too have these tires with 23k miles on them, and for the last 1,000 miles I've been dealing with significant rumbling/vibration at 15-20mph. This morning I took it in to the place that sold me the tires, and was told that they were badly cupped due to "poor alignment" and "the sports car suspension" (bone stock '04 WRX). Funny thing is, they hadn't even checked my alignment when they told me that. Apparently, an Impreza is too aggressive to put these tires on
They were supposed to check and call me back. I have yet to hear from them. I only rotated them every 9k miles, but it's a daily driver and is driven with a very light foot.