Just maidened my MiniMag with the original Multiplex floats. Flight behaviour hasn't changed much. The additional weight is probably more noticeable than the extra drag, but the MiniMag still performs really well.
Looks really strange when you do rolling circles with floats or low inverted passes.
Haven't tried flying from water yet, but tested the other extreme: The model stops on dry short grass in approx. 6-7 feet - without overstressing the landing gear, or causing damage to the water rudder.