I put Motad on mine, but it ran weak, so I had to sleeve the outlet pipe down (it's an easy job).
I can give you the dimensions if you want.
After that it ran fine, and they've been on for around six years now, and thus far no problemos.
However other Fazer owners say they haven't had this problem.
Maybe Motad have altered the outlet pipe since I bought mine, or I was just unlucky.
One other point: don't over tighten the header nuts as you will bend the stainless flanges.
And finally, I've got a set of original downpipes, that have no corrosion on them whatsoever, if you live near Bolton and you don't mind parting with a few quid you can have them.