However, what about flat water paddling? Can a heavier guy like myself get away with having a lower volume ski or would a heavier paddlers weight just kill the intended/optimum waterline?
I'm 100% certain that it makes very little difference on flat water. For that matter Oscar Chalupsky weighs 100kg (on a good day) and is still one of the fastest paddlers in the world on flat and downwind on a Nelo 560 which doesn't have much more volume that your 550.
A difference of 10 or 20kg makes hardly any difference to the waterline. You weigh 88kg in real weights (!). I was at 90kg two years ago but have come down to about 77kg today (simply by doing more exercise) and although the boats feel a little more agile, that's partly down to being fitter.
As for downwind - my paddling buddy weighs over 90kg and he scarcely gets wetter than I do on a downwind. We did notice a difference when trying out the Nelo 520, which is a very small volume boat. I found it very wet i.e. a lot of water came into the cockpit and had to work hard to keep the nose up, he was just about underwater.
I think you'll find that 550 has a tendency to bury its nose downwind (which is why all the Nelo boats come with the wave deflector as standard equipment), but the one time I was able to do a downwind on a borrowed one, I also found it a wet boat. On my paddle it didn't have the wave deflector installed and a lot of water came over the nose and into the cockpit.
Rob
Currently Epic V10 Elite, Epic V10 Double.
Previously: Swordfish S, Evo II, Carbonology Zest, Fenn Swordfish, Epic V10, Fenn Elite, Red7 Surf70 Pro, Epic V10 Sport, Genius Blu, Kayak Centre Zeplin, Fenn Mako6, Custom Kayaks ICON, Brian's Kayaks Molokai, Brian's Kayaks Wedge and several others...