Id really like to try and help you do this right, because when things dont go as expected, vendors sometimes get discouraged from offering more parts.
See my previously attached pics. I built the same exhaust(it was beautiful i might add) and what i got was no power gain, loud sound, and $1500 or so lost. Amazing similarity i might add, im pretty sure its where your customer "drew inspiration" from.
I also bought headers, and found out the same thing the hard way, 5 peak power, 8 torque lost, banging the frame, and another $1000 gone.
You are also going to find many people who say they are interested will not carry through with purchase when you build it and post the price. It took me being on here for a year and going down the same road to figure this out. If you decide to do this then more power to you, again im just trying to help. Before and after dynos (with no other variables changed) will be a must, and if substantial gains are achieved maybe itll be worth you building.
Also, a lot of us dont like our car being louder. If the cost of the exhaust vs the headers is about the same, id much rather see a set of headers that actually makes horsepower without loosing torque, than i would an another exhaust setup...besides, exhaust tone is so subjective. Equal length headers would be awesome but id recommend you also experiment with shorty style and log style and dyno and datalog the differences.
Please make sure they fit properly and dont require banging them in with a hammer for them to clear the frame! If it at any point in time becomes necessary to modify the stock exhuast down pipes for
dyno-proven headers to fit, i dont have a problem with that.
I can give you more ideas if you like. Its just good to have another vendor willing to try and offer us parts.