That 8-15 pound range is for the airplane without the floats.

Like anything else you add to an airplane, you want to use the lightest floats that are big enough to do the job. Floats also create drag and some lift in strange directions. The smaller the float, the less the drag and off center lift.

The correct size floats will add about 2 pounds. Your 4* flies very well at 7 pounds. It will still fly pretty well at 9 pounds. Do you really want to see how it will fly at 11 pounds?

Jim

BTW, your choice of float plane is excellent for the reasons you stated, plus a low wing will be more resistant to crosswinds than a high wing would. Your problem is that at 11 pounds, you no longer have a light wing loading.