The bolts holding my cats to my headers (or vise versa) were nuts and bolts and loose (not welded to the cats). Because of the intense heat, they do get brittle. Hitting them with some bolt loosening stuff and letting it sit a half hour ain't a bad move.
The passenger side header is a lot more work than the driver's side. You have to remove the cross-brace to get to it.
The size difference between stock and aftermarket is huge, huh? That's what gives you the power increase that makes you feel like you are using 109 octane or something along those lines. My 2.5 feels a lot faster with my aftermarket headers and exhaust on it. I have a little more power and its a lot louder. The combo is simply amazing with the top down, especially when you go under a freeway overpass!
Oh yeah, be sure to zip tie off the AC line on the driver's side to move it away from the new header, and then wrap it with high temp foil tape or exhaust wrap to keep it from melting.