Canadian Joy hit on some of the major strong points of the Glasair Sportsman 2+2. The wing strut behind the front doors is good for giving something to grab and having the strut behind the door makes getting in and out of the aircraft easy. Plus on float planes, you usually have to stop the engine, jump out on the float, and run to the front of the float to catch the plane from hitting the dock. Yeah, there's more aerodynamic drag with wing struts. But you usually end up with a lighter aircraft and no "carry thru" spar in the cockpit.
Another strong point on the Sportsman is the ability to switch between a tricycle, tailwheel, or float landing gear. The Sportsman allows you to do that relatively easily. The comment about "popping" the manual flaps is a good one. This technique is also used for float flying.
As far as a TW version, I would seriously recommend the T3 tailwheel as an option for the RV-15. Once you've flown with a T3 tailwheel, you'll never want to go back to the standard TW.
As far as a cargo door is concerned, one thing I don't like about the Sportsman "cargo" door is you can't open it with the flaps down. The door interferes with the flap trailing edge. This can be a concern with landing on water since you usually land with full flaps and if you have a mishap on landing, the person in the back seat might not be able to exit the aircraft. And as far as the size of the cargo door, the bigger the better. Look at the cargo door they have on Maules and Bearhawks. Yeah baby!!!
I also second the comment about large, removable inspection panels. It makes maintenance SO much easier!! I also agree with the comment about extra long-range fuel tanks. With extra fuel capacity, you have the option of either lots of fuel or lots of cargo. Your choice. The tip tanks on the Sportsman are nice since they are used to fill the main tanks. There's no problem pumping the tip tanks "dry" since they don't directly feed the engine. One more thing to note here. The Sportsman has small "header" tanks in it. This is to make sure you don't starve the engine of fuel if you're doing a hard slip on landing with low tanks. Yeah, it has a "Both" feature on the tanks, but the header tanks make sure you have some fuel if you end up uncovering a fuel pickup on landing.
And on a completely different note, maybe after Van's introduces the RV-15 they could get into the kit "float" business. Aluminum float design would be an natural for Vans since the design of aluminum floats is very similar to that of aircraft designs.
I hope this helps.