I bought hand made headers for my z900 one year ago. Combined with akrapovic silencer (with dbkiller) the loss of torque at low rpm is really noticeable. The sound is incredible: clean, nice and not so loud not so silent, perfect. The problem is the lack of torque at low rpm.
I really miss how good did work the bike at low rpm when I had stock headers installed, but I hate the stock sound.
I don't know if akrapovic headers would solve this lack of torque at low rpm. The problem is they are so expensive.
Right now I swap between original headers and handmade headers every 6 months so I can enjoy the best of both but I would like to find a final solution. Not sure if there is one.