Angelo, thanks!
here's the video of the HFC comparison.
http://www.youtube.com/watch?v=0tLVISAlSKg
there were issues pulling the old cats off, then issues with the Z1 HFC (O2 bung snapped clean at the weld, easy free fix at a local weld shop).
coilovers go in this weekend.
driver side bolt would not come out, finally came to the conclusion it was heat welded from the high heat of the header. the angle at the driver side of this bolt is insanely inconvenient and difficult too. this was on the driver side, passenger side came out with no issues. i had the trickiest time finding a way to get my die grinder to this bolt. once i chopped enough off, i slammed a 13MM socket onto it (real size of bolt head is 14mm), and then broke the nut loose easily after this.