Great video + great sounding combo. I have the same "hiss" noise with my Akrapovic/Berks combo and to be honest I would sooner not have it (the hiss noise that is), although I think it might be a side effect of fitting HFC's.
Yup the hiss is the air going through the cat tubes
and thats the very same noise that I love - just goes to show that everyone needs to hear personally the options before deciding