We had a good snoop round the ones on display at donnigton, on the Magnaflow roadster.
The quality of the brackets is not the issue, it is the doors on the Z, that cause the "wobbly effect".
They dont have an inner solid frame apparently, which means when they are suspended in the air like that, they can wobble from side to side quite easily, which puts stresses on the mountings that they shouldnt have to take.
The doors on the Z are very long too, which adds to the effect. Unfortunately they do have to open quite wide as well as go upward, which negates the "practicality" pottential somewhat.
If they went straight up, without having to come out very far at all, they would be quite practical.
For now though, they just ook quite cool!