You'd be better off just gutting your stock cats and getting the spacers people have used for the rear O2's. You'd probably get more power and it'd cost you $30.
If you were adding them plus the Z28 cats no you won't need a tune but that's because it aint changing much, laymans terms for not a lot of added power. Looking in the GMPP catalog it doesn't include the cats though which means modifying existing or buying very expensive GM cats. I also only see an adapter kit to put the Z28 catback onto the LS3 cat connection point but not the other way around, tri-Y to factory exhaust.
http://www.chevrolet.com/performance...parts/z28.html
Buy headers.
