I have TTP headers and high-flow cats on my 2.7 Boxster which I like a lot, but they are very expensive. I think I would recommend the Fabspeed header with sport cats, you could then bypass the second bank of cats with their bypass pipes.
In my experience, the headers added some sound, especially at high rpm's, but it was less than I thought. The indi shop I was using said in their experience a header/cat rig on a 2.7L would not give much of an increase in hp.
That was my experience, I think it was 5-7hp increase on the dyno, something like that. Also in my case I was chasing CEL lights for about six months after we installed the new exhaust system. I have resolved the CEL issue but it was a quite a pain at the time,
Ed
