Is someone who owns the performance mx able to confirm that the middle scroll/mouse click acts as an actual mouse button rather than a change from mechanical to smooth scrolling?
As someone else has already stated - yes, the scroll wheel is also a button. However, it's overly sensitive (even with smooth scrolling disabled), so don't expect the same sort of middle button control typical with previous gen products (I also have an MX518, and the scroll wheel is a much better button).
Also, if you have terrible (jerky or non-responsive) scrolling performance then you most probably have a dud unit. Just RMA it for a replacement.
The one we purchased brand new had this issue, and at first I thought it was actually how they performed normally.