What I would do is this.
Save the cat back and acquire parts.
From what Ive seen the best mods are vararam or S&B intake, Ported TB, oil catch can, LT headers and tuner. LT headers are not really easy to do as they are $$ and you have to get a cat less ypipe or get something custom so thats not a real option for many of us.
Since you have the cutback i would acquire the vararam, TB, and oil catch can. Then order the tuner from Matt and get the custom tune for your mods. Install everything and get the full impact of the mods.
But as stated hemi fever will retune your custom tune for mods down the road as long as its the same truck. So getting a custom tune for your truck and then sending him the tune every time you mod is an option. but way less satisfying than doing it all at once.
I think I want to quiet my muffler down (18" jones max flow) so looking at a dynomax catback as i think the 23" muffler will help and gives me no resonators and larger 4" tips.I already have the oil catch can, picked up a used vararam. thinking the cutback and a tuner and Im all done. Besides maybe a V6 Efan and 180 Tstat.