I just went through this with my 999R. In fact I bought the exactfit belts for it. The day after I received them I had second thoughts and ordered stock belts from ducati. I just put the stock ones on.
The exact fit are well made and almost exact replica. FWIW, in closely examing the new OEM from new exact fit the only diffrence I found was the exact fit seems a very small bit thicker (not wider, thicker) in the belt portion, teeth and spacing seem the same, and the teethed surface is rubberized vs. the OEM. Otherwise exactly the same spacing etc. What any of that means in terms of longevity, stress etc I have no idea.
Second FYI, comparing a droid phone with application 'gstring' and to a device to measure belt tension with an external microphone I got exact same consistent readings.
I still have the exact fit belts in a bag.
2006 999R Xerox