I think 15 - 20 hp can be expected without a tune. If you live in an area that does not require emissions you can skip the cats...I wouldn't either way, just cause it is a little better on the environment and I like our environment.
LT'S with a tune, new exhaust, CIA, 180 thermostat could maybe get you 30 HP. (rear wheels) That is how you must judge you mods. If you are going to put that kind of money into your mods. A day on the dyno (if you can find a shop) will pay for itself. Closest to me is in Columbus Ohio 3 hours away.
i dont really know what hp points i got from my system but the headers were listed to give me 17 to 20 but i got it tuned so more than that just dont know by much but i love the performance and sound its given my truck.
My long tubes netted me 21 at the wheels (before #s 308/316 and 329/347 after) Dyno was two days apart. DA and humidity were less than 3 pts off and temp was +/- 1.5*s.
I am on a 4th gen though.
I say do it. Its a great mod for the trucks and IDK if youve ever seen stock hemi manifolds but they are restrictive as hell! Cats are a personal thing. IF your in an area that requires emissions testing i wouldnt skip it. Im in SoCal but my truck is registered in MI so i passed on them, but I have a way out for now with the out of state thing.
Yep 180 stat may help offset from headers being installed since it may go lean. Got Hedman LT's and 3" ORY dirt cheap from a parting sale of a local member years ago. It was very noticeable during WOT. Later removed the joint section of the ORY and replaced it with a Magnaflow Y.
LT's >> Y- Pipe >> No Cats >> non foulers >> Dynomax Superturbo dump after axle.