Couple of comments.
Heavy Air
I sailed my HT this summer in some really heavy winds, 25-30knts, on a protected inlet on Cape Cod. Flat water, and I never took the spin out of the snuffer. It was one of the most fun moments I've had with this boat. I let the traveler all the way out, and was sailing on a deep reach, doing the wild thing like the Acat's do downwind, with all the controls on the main “off” – ie no downhaul, rotator all the way out, outhaul eased. It was as fun as flying the spin, but less complicated and the boat really flew, it was a lot like the way the Acat will go downwind on a hot downwind angle. If you are just interested in going out sailing in heavy air, this setting is a lot of fun.
Upwind in heavy air, put every mast/main control on full “on” setting. Ie, pull the downhaul and main sheet as hard as you can (do this either together or do some sheet and then some downhaul, but don’t do one all the way and then the other.) I have a replacement Kevlar Main that Peter Cogan made me and I can turn the main into a board and upwind in heavy air is very manageable, and very fast, as long as you can get the main very flat. If your main is old and stretched out, you might not be able to get all the curve out of the main and you won’t approach this sail setting. The mast will do this nicely if the sail is cut right and you apply enough downhaul. I think this one of the boats strongest features. I have a 16/1 downhaul and I can really hammer on it.
Widening – and devaluing.
If you keep the standard beams and tramp when you widen the HT to either make it more manageable or make it a 18sq, I don’t think you’ve devalued it at all. You now have two boats for the price of one, and since there is zero class racing for the HT, who cares anyways? If we do get back together as a class, I would propose 8’6” wide for US HT’s as a max.
Altering the main
I wouldn’t alter a 20 sq HT main to make a 18sq main, unless the HT main is trashed, and then you could experiment with hot knifing it down by 2sq, but you’ll probably want a sailmaker’s input. I would go have a new 18sq main made, if you are serious about 18sq racing, you’ll need a 18sq main made for the HT mast, which has very good bend/flattening controls. A new 18sq main would be well worth the investment, AND it would/could be a heavy air sail.