I have learned much from my customers as far as mods on the 5.7. It's in the trucks, and I have converters in 8 out of 10 of the fastest Hemi trucks in the world. We are able to see mid 1.5s for 60 foot times... Thats a quad 4x4.
Best mods for the buck without touching the engine:

CAI+Tune, and exhaust. I think you will see gains on any higher flow system, but I got zoomers for the sound, and lack of drone.
Oh, and the Converter if its an automatic.

The Diablo tune is great, but a custom tune got me 11 more HP. You still need the tuner, but Dyno Steve from RDP Motorsport Australia will be down to visit in 30 days and I bet if we get enough Challengers together we can get a great deal on his time. He will be in Ca, FL, DE and OH in the month he is down. He will spend his most time in DE at his USA division. Steve has the fastest 300c in the World, and has worked on most the top 20. Like Ron at Zoomers, he is real people. I can get a deal on dyno time in a few spots through out the US.
You manual guys will see more to the tires on the dyno, but we Auotmatics have one thing you wont. STR (torque multiplication) at launch. You release the clutch and your 1-1 when it fully couples. The automatic can be set as high as 2.45-1 before you start to drop in MPH on the 1/4. I like a stick, but autos are the future in racing...

I did the valve body in my transmission, and documented everything. I was able to back down on the tune some after some changes I learned for an old German racer.

I like the R/T rim size! Ever try to get a sticky set of tires in a 20"? Not a lot of choices. But 17 & 18" have many.

Most who mod the 5.7 for real gains also swap the cam (6.1) and do head work

Lets see if this will even post