RE: iSonic105 technical ride
Other than my trips to the Columbia River Gorge and the Delta (east of SF Bay), it's pretty rare for me sail in fresh water. But, even when I have, I really haven't noticed much difference in my ability to plane. However, I'm aware that inland locations can have very different wind, especially if its dependent strictly on thermal conditions in combination with certain types of topography.
I understand your point that a bit of extra volume is your friend in gusty up and down wind. Yet, for myself, I view volume more in terms of simple float rather than the key to early planing ability. In my mind, the character of a board's rockerline is what makes the real difference. Of course, most high volume boards tend to have flatter rockerlines that promote early planning.
The real question is how big to go. My limit is about 120 liters because that's tons of float for me. Really, 100 liters offers me excellent float, and it readily supports a 7.0 sail, so it's a more desireable choice most of the time. But, if I need to go with an 8.3 sail because of real light winds, I need the bigger board and fin to support it. Given the fact that you have set your sail size at 8.5, and because your focus is not oriented towards a FW upwind/downwind approach, I doubt that volumes greater than 120-125 liters will not buy you much.
Another way looking at the situation is in terms of maneuverability. Sticking with just the needed volume, I find that I get to maximize maneuverability and nimbleness. I guess that is why I recommended that you experiment a bit with the iS105. You might be surprised.