I don't keep that much of an eye on my MPG but from personal experience on my Ivanised Fazer there probably isn't that much difference if you keep a steady hand on the throttle. With an Ivanised bike though there is always the temptation to explore the newfound power and delivery which is most noticeable in the upper reaches of the rev range so I would guess that folks who say they have noticed a decrease in fuel economy have actually changed their riding style.
/Edit: SLG beat me to it.
/Edit: SLG beat me to it.