Hi Andrew,
While I know it's an Illinois forum, I'm actually from "up north" in Wisconsin. (Hey, there are no more windsurfer left in Milwaukee, so I need to hang out with you guys!) I sail out of Racine, Sheboygan, and up in Door County. I also sail Winnebago on West winds, which is a surprisingly fun lake that develops solid swell. There are still a handful of truly excellent sailors from the Madison area, but I'm definitely looking for more people to ride with.
There's no doubt that volume helps when it's light, variable, or on-shore. However, for those days when the wind is more solid and conditions are bigger, nothing flatters your riding like a smaller board. They are less bouncy, offer better control when overpowered, turn infinitely better, and are an absolute joy to jump. What's not to like?
There's something almost romantic about the idea of a 1-board quiver, but I honestly suspect you'd have a much better time on the really windy days on something smaller than a 105. For me, 100+ liter boards start getting pretty bouncy around 5.5 or 5.0 conditions. For anything windier, I'm having more fun, and riding much more aggressively on something smaller. The biggest advantage to me is how smaller boards turn. I feel like I can turn any radius I want, whenever I want, on any wave I want, whereas on bigger boards I feel much more limited. (I've never spent much time on big boards with lots of rocker, so I might feel differently after a great day on a big wave board) You owe it to yourself to pick up a used sub-85 liter board and give it a shot. I think you'll be glad you did.
Just my two cents...