the only effect moving the cat further down the pipe will have is a slightly slower cat warm-up time, so it wouldn't affect sniffer tests. if you moved it to the back of the car, it'd never warm up enough to function, but even a foot or two is fine.
some OBD-II cars have a 'slow cat warm-up' diagnostic routine which COULD be tripped if you relocate the cat. lucky for you, it's not mandatory in the OBD-II spec, and because GM is lazy, i'm pretty sure the LS1 computer doesn't do it.
some OBD-II cars have a 'slow cat warm-up' diagnostic routine which COULD be tripped if you relocate the cat. lucky for you, it's not mandatory in the OBD-II spec, and because GM is lazy, i'm pretty sure the LS1 computer doesn't do it.