i would get the stainless one for a powerlog.
for the downpipe get a 3", but the cat depends on you. i personally would keep the cat, but that's because i dont like having smelly, dirty exhaust.
i wouldnt drop anything lower than a 3.5.
just remember, this stuff is not the end all and be all. youre going to need a tune to take advantage of it all, which cannot be stressed enough.