robr298,
I first installed a K&N CAI followed by the Maganaflow Cat-Back (Completion Series) Exhaust System and noticed an improvement, but not spectacular. Last week I purchased a Diablo Predator and installed the 91 Octane Tune (93 Octane is unavailable in my area). The Diablo 91 Tune made a significant difference in terms of pulling everything together and letting the engine run harder, yet much smoother. I can't say I have more bottom or top end, rather my Challenger runs stronger throughout each gear with the 91 Tune. I have also picked greater fuel mileage/efficiency. No doubt others may have a different opinion and my observations are strictly subjective, as I have not run my vehicle on the dyno (before and after). Nonetheless, I believe you will not be disappointed and it’s well worth the money.
P.S. My 0-60 time improved from the low 5's to my very best 4.5 seconds (Auto Transmission)