Welcome to the Breeze.. and I'll give you my thoughts..
IMO.. You're better off with the 5.5 sail size over the 6.5.. mostly for the shorter boom.. lighter rig weight.. and it would also have a bigger wind range..
l've tried these new SUP rigs and found them to be pretty good.. They are mostly designed for light wind sailing.. and light wind wave sailing..
I think if you intend to sail overpowered in stronger winds and be fully planing I'd go for a normal type windsurfing rig..
I'd recommend trying one of these new lighter and cheaper SUP rigs for the conditions that you say you will be using it for..

Hi Bird1962. Welcome back. Which state are you in?
Here in WA, I would not go bigger than 5.0 for Wind SUP (AKA Bimbling).
As DJ says, if you get to planing wind strength, a dedicated windsurfig board is the go. There is usually way too much tail rocker in a SUP to plane well. No straps makes it pretty hairy as well.