I don't know about sail area to beam issues, but in a recent Multihulls Magazine article, a designer (and I forget off the top of my head who this was) related that for catamarans, there is an ideal length to width ratio of 2:1. The claim being that more than 2:1 (like a 20' x 8.5' boat) doesn't have the ideal righting moment, while less than that(like a 20 x 12+ boat) leads to a potential for pitch-poling. The designer was talking about big boats, not beach-cats. But at 20' x 10' a Tornado would seem just right. That all doesn't take trapezing into account, of course.