Having the drummers kit on a drum platform on a floating floor really helps. Unfortunately a floating floor is not enough to do the job right. A floating room needs to be constructed. To float a room, first new walls need to be constructed. Walls can and should be built off the existing floor not the floating floor.
Air is one of the best soundproofing materials.
Metal studs can be used. For extra support isolators can be used to support the walls off the existing walls. After the walls are done, you can then build your floating floor as mention above. The last step to a floated room is the ceiling. The ceiling must be suspended of building isolators.