Yes, I flew a couple sets of 4580 amphibs on Beavers. Yes, they are limited to stock GW (5090). Ours were modified with Cleveland brakes/wheels, and equipped with up locks, which the originals were NOT. And, yes, there's a reason I asked for up locks.
They are very simple, systems wise, no electrics, no motors, nada.....just a hand pump. No backup system.
They perform very well, though if you load the plane heavy, they sit low in the water. None I've seen have float compartments, but it'd be simple to install a couple of the EDO 4930 forward compartment hatches. The 4930s were available with a main compartment hatch, and one forward of that. So, those forward ones should work fine on 4580s.
Our outfit had at least one or two sets converted to "4930"s. The conversion was done in Seattle area, but I don't know who did it, and I doubt anyone who was around then knows either, but Kenmore might. They did this to go on a "homemade" Turbo Beaver, which was not Certificated. Could this be done legally these days??? No idea, nor can I even guess how much it'd cost.
One thing you need to understand regarding those floats: The main wheels retract into the compartment just aft of the step. That's "normal" for amphibs. What's NOT normal for amphibs is that there is NO metal housing around that gear. The tire literally presses up against a rubber bladder, which seals the compartment. When the gear lowers, the rubber bladder keeps critters out. Those bladders should be inspected regularly and replaced occasionally. If one splits (and yes, been there), that compartment floods REALLY fast.
They are great floats in many ways, kinda small to operate at the weights most folks are operating Beavers these days, but if you don't need to operate really, really heavy, they work fine. And, yes, I may have operated those airplanes in excess of 5090 once or twice.....
The perform well, and are tougher than hell. I had a main gear come out of the well as I touched down right next to shore once. Plane did an instantaneous 90 degree left turn (I distinctly recall looking out the right window and seeing the tie down ring on the wing tip just grazing the water) and slid up on the (VERY) rocky shore, all the way to the steps. Six of us pushed the thing back in the water, I assigned one of them to gently maintain pressure to keep that main gear up while we took off, and flew home. I may have thought about a ferry permit.....it was right after that the floats got up locks for the mains. Upon inspection, the keels of the floats were bent over about 70 degrees from the rocks, but no damage to bottoms.....those things are TOUGH.
If the nose gears come down in flight, you have more problems than that. And, it's really easy to SEE the nose gears. I was told by our Maintenance Chief that he actually watched one of our pilots land one of these planes in Lake Hood with the gear down......apparently he got it slowed really well, very nose high. Chief said the spinner nearly touched the water, but it flopped down.....he never said a word to the pilot. This happened after hours, so the Chief just quietly went out the back door, got in his car and went home. Did a VERY thorough inspection of the plane next day....no damage at all.
