I had my local shop put on a set of JBA shorty headers. I noticed some HP gain, but nothing amazing. I was really disappointed with the sound. I was expecting a much deeper/throaty sound. It improved the sound at idle and at moderate / aggressive acceleration, but at WOT it is very loud and raw.
I am not unhappy with putting them on, but if I had it to do over I would of rather put the $750 toward the stage II Comp Cams which is my next mod.
After the CAI / Tune & Axel Back Exhaust you really don't have any other major WOW mods that are relatively inexpensive. Long Tube Headers which require a new mid pipe will cost north of $1K. Cams will easily run north of $1K, Forced Induction $5K+
Regarding your original post regarding installation: I like tooling around my car and I have a 67 Mustang I am restoring, but for my daily driver I personally feel $300 to have the installed by a pro is worth the money/time/pain I would of saved by doing it myself. Without the proper lift not a fun garage project, but I take my hat off to anyone who has the courage to take it on!