If it comes with one, it will only drive the price up, you can get them anywhere for less than $20, its not a big deal to buy one.
A minumum 17rwhp/tq gain is nice indeed for a quick bolt on with the CAI, Exhaust, Diablo... 1. As far as bang for the buck status... what is a BALLPARK price range? *I'm into sweet deals/bang for the buck mods, as I'm financially challenged. 2. Would high flow cats lend even more hp/tq given the CAI, Exhaust, Diablo setup? OR would it harm the TB performance (not enough pressure)?
Nah if you have the Diablo to delete the o2 sensors then go head and either run high-flow ones or um "loose" them, you have extra air entering the engine with the CAI and TB so a little more escape path will do you some good.