One thing about the off road vs. high flow cat is you lose 'tuning' on the exhaust with the off road pipe. Air pressure waves bounce off the surface of the cat and actually help 'suck' more exhaust out of the cylinders, effectively increasing airflow (=more power!). This is best at low speed. At high speed, the cat restricts the airflow, so you'd be better off with the off road pipe in this case. So it's a trade off.

Let me know if I wasn't very clear, I'll try to explain better if this didn't make sense.

EDIT: I realize you guys are talking about what bolts to what, I don't really know about that stuff though. Just thought I'd throw in my .02 - the off road pipe isn't necessarily as good as you might think.