oem has a resonator. personally, I'd buy the 30 dollar wrx setup and hack it up, so if you screw up you still have the factory exhaust to fall back on, or put it on for resale. if I did the work for you I'd weld it, some muffler shops are fairly cheap as well. I noticed a slightly deeper sound with the 2.5in but it seems to backfire more( or maybe its just more noticable) either way sounds excellent, and I know because I've had both. plus a few more.
and as far as staying away from honda sound, bigger pipes, more packing in muffler ( dynomax over a can or glasspack) and keep the damn resonator. I tried eliminating the resonator in conjunction with a flowmaster super44, and it sounded just like a civic, I wanted to shoot myself. glasspack resonator is better than nothing, but it still doesn't beat factory
joel always says I'm the guy to ask about the exhaust on these rigs. done more playing around with my na than anyone should