I've had my IS3 since OCT of '03. I've had 2 different sets of headers on my car, OBX (w/stock Y-pipe cat) & currently Dezods reds (no cats). I just had my Dezod reds installed a couple weeks ago w/a set of BC 264's. Despite everything I've read on different threads regarding how loud your car is w/o any cats, the Dezod reds are not THAT loud. I gotta friend @ work w/a no cat set-up on his RSX. Now THAT'S loud! I was expecting THAT volume; & actually would have been OK w/it. But, it's surprisingly not as loud as I expected. Maybe 'cause I kept my stock resonator on, I don't know. But, I luv the sound. It's perfect.
So, don't think if you get the reds, that you're gonna break sound barriers or have an untollerable volume level. They're not THAT loud. The sound couldn't be more perfect IMO.
I actually, bought another IS3 last SUN, '04 Sport Design w/70K miles. I plan on doing more subtle mods to that car (for now anyway) & order a set of Dezod greens. Not based on the noise level so much, but the odor of no cats. The Dezod greens are backordered right now though.