ORIGINAL: BlackS4

Yea, so now I get my KO3 replaced, and I thought I would have a little more time to save up for the KO4s, thats eventually gonna happen. Now that the motor is pulled out I am getting timing belt done, water pump, tranny and motor mounts and other stuff that is required. A friend of mine is a tech whos working on the car every day after work.
So once time will come for redoing all this again and replacing turbos with KO4, what I understand is that I'll have to do ECU setting for upgraded turbos, and also I need at least a catback, upgraded diverter valve, and with these how many horses at the crank would I be running? I'm in San Diego where highest octane pump gas is 91.
Also, what do you guys think about Borla exhaust for S4? I believe its an american product, would that be a good choice, or maybe should I go for a Milltek or Remus catback? I dont want the car to be too loud, but some exhausts, such as SuperSprint, are not really satisfying soundwise. I had one on my VR6, was nice but with windows rolled up I barely could hear anything.