Ken, 

The Russian company Beriev had this interesting amphibian at OSH
Look how close the wing is to the water for ground effect
and the length of the  wing strakes. The Russians know a lot about
flight in ground effect. It was built with aluminum so composites
ought to be tougher and stronger.

Look how fast it is with only 420 HP.
I think the payload number is a mistake.
That is probably less fuel.

http://www.beriev.com/eng/be-103/be-103.html

I think the idea will work on the Coot.
You could do away with the sponsons and their drag.

Paul Lamar
 
Aircraft Performance 
           Take-off weight, kg
                                                                                2270  [5000 pounds]
           Payload, kg
                                                                                385  [847 pounds]???
           Engine type
                                                                                TCM-IO-360ES
           Number and power, h.p.
                                                                                2 x 210
           Propeller
                                                                                three-blade,
                                                                                variable pitch
           Maximum cruise speed, km/h
                                                                                240  [150 MPH]
           Operating altitude, m
                                                                                up to 3000 
           Service ceiling, m
                                                                                up to 5100
           Maximum range (with onboard fuel capacity 245 kg), km
                                                                                1100
           Take-off speed (ground/water), km/h
                                                                                137/130  [85 MPH]
           Landing speed (ground/water), km/h
                                                                                129/131
           Take-off distance up to H=15 m, G=2050 kg (ground/water), m
                                                                                440/650
           Landing distance from H=9 m G=1950 kg (ground/water), m
                                                                                460/570
           Seaworthiness, sea state
                                                                                2
           Depth of water reservoir when operated, m
                                                                                1.5
           Cabin length, m
                                                                                6.0
           Cabin height, m
                                                                                1.82
           Cabin width, m
                                                                                1.55
           Volume of luggage compartment, cub. m
                                                                                1.33
           Crew, prs.
                                                                                2
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