So I'm just over 1 week installed. I went over a series of very high speed bumps today. I went slow, and there was no problem with clearance. In fact, the front spoiler guards (or whatever those black pieces are called below the front lip) scraped a little on the highest bump, but there was no scraping whatsoever on the mufflers.
Keep in mind I'm stock height with 18" rims. But I just wanted to post for those worried about clearance...No worries!
__________________
Rain, snow, sleet, hail: 2009 Mazda 3 sGT HB
Everything else: 2009 370z, PW Touring 6MT