Hey @
bjun I'm going to have to agree with others. The stock cat will be fine for some time. just don't know how long. @
Sportsguy83 is right about the sound. The turbo is a giant sound diffuser / resonator whatever you want to call it. I wouldn't worry about it being more than a few decibels higher, maybe 10ish? Just a guess.
I'm planning on replacing my stock exhaust manifold too. If you're worried about smog (not sure the requirements there), you can simply reinstall manifold and load your current tune file before you go. If you're worried about blowing smoke and getting ticketed, well... racecar...
Yes you can install a cat post-turbo. You'd have to take that big 3" pipe after the turbo and have muffler shop cut it and install.
My vote - PTuning header ;-)