Jim is 100% right. A properly designed float will actually improve a lot of the aerobatic capabilities of most aircraft.
The drag they create is actually a good thing as it can slow a maneuver down enough to make it more visually pleasing.
The extra weight on the bottom of the fuse helps to create a pendulum effect which makes the plane more stable both in regular and inverted flight.
One of my old Goldberg Eagle 63's would barely fly inverted. It had the power, (OS FX46) but the wing just was not designed for it. A set of floats made the plane a dream to fly inverted. The positive angle of attack on the floats actually helped in inverted flight. So much so, that inverted touch and goes (dragging the tail through the water) was a regular stunt for us at float flys.

Peter