Is that the all up weight? Does it currently have an engine on it? Seems a bit heavy as Sig suggests 5# for AUW. However at 6# you're going to need 100W/Lb. at least if not a tad more for floats, floats create more drag than wheels. If it were my plane I would consider at least 700-800 W in and considering the efficiency of the motor just how much actual power you will have to produce thrust. You can always reduce the throttle to a comfortable cruise speed.
You may want to check out Inno8tive Products Innov8tive Designs/Scorpion products
They provide battery/ prop charts for their motors which will give you an idea of how much power and thrust each motor produces. These are not estimated but actual tests.
I would consider a 4S pack as well. Consider how much prop clearance you will have with the floats.
Hope someone who has actually flown a Mid Star on floats will chime in.
Out of curiosity, what floats are you using?