I second what AJP said! My personal recommendation would be start with a tune, and if you still want more, then downpipes as the next step. I had an AJP tune for a while (on its own), and loved it, then added downpipes, and some other mods, and made significant power with my 2015 SHO PP. l actually bought AJPs downpipes after he got his truck.
The tune is definitely the greatest difference you will notice. It is also cheaper than the downpipes, and needed to properly utilize downpipes anyway!