If the mast is aluminum, and cash is no problem, you can have someone taper the upper portion of the mast to reduce windage. Assuming you are going to use the 5.2 hulls with the 10 foot wide beams from the Tornado, you "could" leave it at 31 feet but remember that it is a lever trying to pull you over into a capsize or pitchpole.
Are you running a standard 18 Square sail - or the 5.2 main? Depending on answer, I would reduce mast length to match your sail - and remove from base of mast rather than up above. Length to fit the sail luff plus about 10-14 inches for downhaul, boom if you mount gooseneck to mast, etc.
The only difference is your 5.2 hulls will be less full at the bow - and a bit less freeboard to water. There really will be few problems with the hulls except downwind, where you may see some nosediving - especially in waves. With flat water and the mast shortened to fit your main, 12-15 knots should pose few problems - after that - (upper teens) you probably will see some "issues" but if you are tall or a bit heavy in weight, holding it down won't be any more of an issue than on the big "T". When I was actively racing, I was about 5'7" and weighed about 165-170. Winds over 18 were a handful for me. Depending on your weight - you could see the upper limits at around 15 - 18 and after that it will be just experience and knowing how the boat handles. Big waves will probably result in a very wet ride.
I only saw one original 5.2 18 Square race, the guy was from Iowa, and he did well except in the light stuff. Never did measure his sail and his mast was probably around 28-29 foot. At the time, he was an old fart - probably like I am today to some - but was also on the small side physically. Jon Lindahl, and others were in the mid-five foot 7-9 inch range and were probably weighing in at around 155 - 175 or so. We also had a few tall guys who did marginally better from the wire in heavier air...... <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />
