Hey Toby, all sorts of issues from heat shielding not transferrring over to a different style mounting flange to the next "piece" of exhaust which includes the flex section and cat. Toss in different length header bolts for good measure......
If your going to the trouble of swapping out headers, dump the OEM cats at the same time with some high flows.
Shoot me an email/phone number if you want some specifics.
My solution was to buy an end to end 2000 GTS and mount that to my car, leaving my 97 OEM system intact and put away in storage.
Waiting for parts return from ceramic coating (the OEM is all off right now), and then she all goes back together.
Roe High Flow Ovals, newer style header, and ceramic coated all the way to the rear muffler. Expecting no heat issues in the sills.....time will tell!
Sounds pretty bad asp with open headers, that's for sure. Will take some sound files for kicks when the new stuff goes on, just headers, just cats, and then all buttoned up.
Then of course dyno results!
see ya,
EDIT, Janni popped in during my reply. I thought you pulled 10 hp attributed to the headers? Or was your increase a total with cats as well?? I will dyno the entire improvement, once again leaving the header gain up in the air!!!