Check out this thread from djdojo: (hope they don't mind me quoting it.)
www.seabreeze.com.au/forums/Kitesurfing/Review/2012-Ozone-C4-12m-and-9m/I probably wouldn't push it much past 30kts myself using a TT and I weigh about 80kgs. I have a 7m for higher winds although generally I don't go out in these conditions as they are usually a bit sketchy (stormy southerlies and big swells) around my area.
For 75-85kgs I the 9m and 12m is a good spread for NSW winds and probably much the same for QLD. Before I had a 7m and 11m and there was too much gap between them. I've heard 4m gap is doable for the C4 but not ideal.