Obviously a float plane is going to be heavier than the corresponding land plane. That being said, on a good aerobatic plane, a light set of foam floats with a light wire struts doesn't add all that much. The extra weight down low does affect some maneuvers. Looping maneuvers aren't affected much. Rolls tend to be slower and the barrel out some. Snaps are slower, as are spins.

I test flew a Mariner for a buddy and have a Sea Monster flying boat. They do all the regular acro well. My Monster does all the regular stuff, including outsides, inverted, spins and snaps.

I have an old, early 3D type plane, a Stuntwagon, with tri-floats that it still a great flier.

Botton line is they only have slightly less aerobatic capability that a land plane. If you tend to build heavy, they will come out heavier. If you keep them light, they are really excellent.