torque

Was reading an article about how just adding a slip on won't affect mpg , although I notice a fairly big improvement in fuel consumption when using the stock can , I can also notice a drop in mid range torque over my slip on , so guessing it's all back pressure related . any thing to add gents ?

I've done some searching and found nothing specific for the FZ1. Does anyone know the torque for the Exhaust Header nuts on the FZ1? I've found 19Nm for some other bikes & will probably go there unless I find something specific for the FZ1.