Had a buddy that had the same exact setup as you have. The 582 of his might have been in need of a tune-up or something but he could not get it off the water. Actually if I remember correctly he had a hard time even getting it on the step. He promptly removed the floats and sold them on Barnstormers.
I think a guy from SC bought them and then he also had them back up for sale shortly after. I haven't flown with the 582 on amphibs but as has been suggested, they may be a little heavy and one would be better with a little more HP.