I've flown the Amphib EX, and it's a decent performer, the 8650s are infinitely better then the 8000s. The down side is the fuel burn per mile is quite high, and the useful load (as with all amphib aircraft) is atrocious. I seem to recall a BEW of a little under 7000, so full fuel and that's it. Not tha lt you usually need full fuel in a van.
My recommendation, if it's doable for your operation, get the Garret converted Van. Ideally, a garret grand caravan on straight 8750s. The performance is WAY more than the 30 extra horse power implies. It's all about the torque, for reference:
208B 675hp 1865ft/lbs
EX 876hp 2230ft/lbs
Supervan 900hp 3100ft/lbs
PM me if you have other questions