15 r/t best mods bang for your buck....

Hey guys. I'd like some input. I bought a 15 r/t a few months ago and I'm looking for the best mods (performance wise) without breaking the bank.
Only a tad over six hundred miles and I have a Mopar cai, jet throttle body spacer, resonator delete, Flowmasters, and a Diablo (which I can't use yet lol)
So just wondering what else you guys could suggest. I was looking into long tube bbk headers with a shortened x pipe but I wasn't sure if they interfere or get in the way of any regular maintenance.
Thanks in advance for any help!!