I recommend a custom tune 1st. Being a 2015 you will need an unlocked pcm. If you dont want to keep your factory pcm then I suggest doing the pcm swap through Diablo. Much cheaper. Its $499 for the pcm swap & Diablo I3. They send you the unlocked pcm 1st, once received you install it then send your factory pcm back. If you want to keep your factory pcm then your looking at $899 for the extra unlocked pcm & Diablo I3.
If your wanting to do headers then you will need a tune anyway so do the tune 1st then headers. I highly recommend a custom tune from Jay Greene. A huge improvement over the canned tune. Are you wanting to do longtubes with cats or without cats?
If your wanting to lower the Belltech kit w/ the Street Performer shocks is an ok start. The truck will most probably wobble at highway speeds w/ just the Belltech kit. Some dont have that issue but most do. To correct that issue you will need Illicit Customs rear control arm brackets which corrects all the rear geomatry that gets thrown off after lowering the truck. Those 2 things will be a great start to lowering the truck. After knowing what I know & going through what I did I absolutely would not install a lowering kit w/o the Illicit brackets. Those brackets are the only reason I'm still running my lowering kit. Before I installed them I was about remove the lowering kit all together due to rear wobble & unsteadiness. Thats a good start if your wanting to lower 2/4. If you want to go lower then the Inez kit is the way to go.
All just depends on the current money your looking at spending.
Here is mine w/ Belltech 2/4, front Bilstein 4600 stock replacement shocks, rear QA1 TS-905 adjustable shocks, Spohn adjustable panhard bar, rear Hellwig sway bar & Hellwig adjustable endlinks, Airlift rear helper bags & Moog F150 front sway bar endlinks.
Also have a Jay Greene tune which works very well. It runs mid 12's on the 22" street tires w/ just mild bolt ons
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