The 5-8 degrees is not referring to the wing angle of attack, it is the "fuselage rotation from level flight". You need this rotation at two times during ROW: First, you need a bit of rotation in order to plow up over the bow wake. You can do this without rotating if you have loads of thrust or a very shallow draught, otherwise it is easier to give full up-elevator to get the nose up over the water and prevent "submarining". The second time you need to rotate is at lift-off. In order for the plane to climb, you have to either go really fast for the wing to create enough lift, or you need to pull up on the elevator. When you pull up on the elevator, that pushes the tail down. If you push the tail down into the water, (A) the water will buoy the tail back up make for some bouncy skipping and hopping, and/or (B) you will lose a lot of airspeed from the drag... which could result in stalled take-offs.
I think the style of mid-wing plane you have will require at least 2" of additional height at the step just to keep the wings and tail from dipping into the water. If you want something that is easily removable, then why not just make a pair of floats, or use GWS floats? Maybe you could use the parkflyerplastics
Twinstar kit and modify it to fit.