I have had 1320s under the Bruce Dunham STC for 1320s on 7CCMs (C-90-8 with no accessory case or starter and everything taken out to make it lighter. Great performing seaplane even with 2 200 pounders on board. Then had 1400s on my Chief also with Don's Dream Machine Engine and I would not try any larger, heavier floats than those. The 1400s worked fine although their design is not as efficient as the 1320s.
I also tried 1400s on a 7CCM and with two on board, it was not easy to get on the step within a reasonable distance unless you had a good headwind. Too much rear CG, which was not a problem in the Chief as even a couple of fat guys were still over the center of gravity in the side by side arrangement.
Many 7EC guys that I have talked to have had the same problem with 1400s due to the weight with the starter etc. I think 1650s would be worse.
After all of that, I believe the Type Certificate only allows the 1400s on a 7DC, there is an STC for the OK1500s on the 7DC, but other than the Dunham STC, I at least don't know of any others, but J.J. Frey I bet would.
_________________Robert P. Kittine, Jr.
West Nyack Aviation, L.L.C.
New York, New York firstname.lastname@example.org