from this post, eons ago:I have no real comment there on how much extra HP they added for my car. What I can tell you is that they seriously smoothed out the HP/torque curve such that my car pulled extremely hard all the way to 7,400rpms. Totally different than stock with the HP curve severly dropping off towards redline 6,900. With the headers it basically flattened out the curve making for a very nice long smooth pull from 3,000~7,400rpms so while some may argue that the "peak" hp did not increase I will definitely have to say the HP at 7,000 was probably 30~40 HP greater than stock because of the lack of a drop off. Take it for what its worth and look up my old dyno charts before & after and you will see the curve is almost flat for a very long time.
Now...your best & cheapest option is to just punch out the CAT's vs $2,000 for a custom set. Never heard that headers add noise/heat/leaks/rust unless of course you buy them at Walmart. With all products....by only the best and you save $$$ and headaches in the long run unless you putting it for show & quick sale to some idiot buyer. If I recall after speaking with Rex a few months ago my headers (now 3+ years old with over 30,000 miles) are doing perfect with no issues with leaks, ceramiic coating, etc...then again they cost me $2,000 and 6 weeks for the idiots to finally build them although apparently they did a decent job.
anyone-making-headers-for-the-g50-t254418.html
in the post his sig states 14.2 @ 100...you're there before headers, high stall etc.
not sure what happened to him - like several others rsiwicki moved on. I believe his car lives on though, sold to another board member - if my search skills are correct.
and that is with it de-tuned some for the street with the 3" muffler on the end of my new design of my 4"y-pipe. Tonight is the 1/4 so if I can keep the car cool this thing better see mid 13s! Lol and at our track I have yet to see any new 5.0 mustang run into the 12s/ all 13-13.4s so that should tell how s*** traction is here. With the new headers that I am making it might not need to launch higher then 2200 as it has traction problems now with DRs.. but I am excited. The stock headers are 100% the reason it has a peaky power band, going to fix that! 
i can still taste vomit in my mouth... jacked up Q45? LOL
more work then I thought originally that's for sure.
