On a canoe that size, especially one with outriggers, I think I would much prefer a mast thwart to a mast seat. It is a substantially stronger installation for a boat with a fairly generous sail (for a canoe). You also have the "flag pole principle" to consider when you add outriggers or go to a multihull design. The outriggers and their buoyancy severely reduce the hull's ability to heel over in a gust and spill wind to reduce stress on the mast and its base. The mast just has to stand upright, like a flag pole and take it. For this reason, the mast, mast step and partners (thwart, seat, etc.) need to be somewhat stronger on a multihull or outrigger configuration than they would have to be on a monohull. Dropping a seat down from the inwale on long bolts and spacers is never going to be as strong and secure as a thwart bolted tightly to the gunwale bottom.
"I've never seen a sailing canoe in action so have no idea how much a canoe heels going upwind. You seem to be saying that the larger the sail, the greater the heeling angle will be. Is that correct?"
Yep, sail size and the height of the rig are the determining factors, sometimes balanced (literally) against how much of your body weight you choose to hang out the other side to counteract the heeling force. Obviously, the weather and wind you decide to sail in are the other major factor. Some prefer to sail in light wind, sitting calmly on the boat's bottom, others prefer more excitement and risk and like to let it all hang out. It is possible though, to add so much sail area or rig height that in your average sailing conditions you are not able to trim the sail in properly because doing so tends to overpower the boat. Sailing around with a sail which is luffing most of the time to prevent getting knocked over or the heeling submarine-ing your outriggers is not very efficient sailing.
As to the sail business, I'm retired and no longer doing it. After nearly 35 years of crawling around on a hardwood floor, grinding my knees up, and following an upper aortic aneurism, it was time to quit.