Of course you can hike out to help balance the boat (like on every other sailboat made), this helps, but you still get severe weather helm at higher speeds because of the tiny rudder on the TI when you have bigger sails. And because the hull is a displacement hull, even hiking out only gets you so much speed before the horsepower required to drive the boat faster goes thru the roof. An lets face it, 90 sq ft of sail is very small for a boat this size.
That problem is kind of a catch 22 with the TI. I have massive sails on mine (265 sq ft) but really can't use all that sail in winds over 12mph safely without tipping over or risk breaking something (though I do it all the time anyway). What I have is really rigged for the typical 5mph winds in the Sarasota area. Plus the weak and smallish rudder isn't really designed for higher speeds. I have had my TI over 18mph on several occasions (in 20 mph plus winds), and if I crank the rudder at that speed the rudder just snaps off. I have a similar problem with the AMA's if I'm screaming along at over 15 mph and a gust hits, if it buries the AMA underwater at that speed the nylon sheer bolts on the AKA braces and the AMA just folds in, I have nylon ropes bracing my AMA's to prevent this problem, but there isn't much I can do about the rudder pin problem.
Because the TI's mainsail is boomless, it tends to put more healing moment on the boat than some other sail designs ( healing moment is the force trying to tip you over). The easiest solution would be to extend the existing AMA's out another foot or so per side (making the boat 12ft wide), by doing that you should be able to increase the sail area about 15% to 20% ( instead of having to furl the sail at 12mph winds, this would now be around 15-16 mph). Of course if you hike out to help balance the boat, all the figures increase even more. "The best solution is to hike out and never let your AMA's touch the water". Since most people sail from the back seat, hiking out from there is pretty tuff to do. I only sail from the front seat, but I have a large bow sprit to get my fore sails further forward, and they are angled so they also provide lift for the front of the boat, this lifts the bow out of the water (reducing wetted area), and helps you get on a plane (actually it's a semi-plane).
I noticed that anytime I have been able to get really good speeds, the tip of the bow is typically out of the water 4-6 inches or so (just an observation on my part), and my AMA's are not touching the water except at the very back, I hike out to keep the boat balanced. This brings up another design issue (the hull) the TI hull is not designed for speed, because it's pointed in the back even when semi-planing, it still drags pretty bad. If the rear of the boat were shaped with a rounded rear (like the Weta), the hull would be able to semi-plane easier. If you ever watch a video of a Weta really pushing hard, you will see the bow in the air, and the helmsman hiked out enough to keep the AMA's from touching the water, if you look at the AMA's on a WETA, they are tiny, non-planning, and not really designed to keep the boat level, you have to hike out (as with most every other boat out there of that style). Also I think when a WETA is really hauling the front half of the hull is out of the water.
Another problem preventing adding more sail area to the TI is the mast holder. If you look at it closely, the only thing bracing the mast forward to back is a tiny 1/4 inch stud in the hull bottom. The hinge point is one foot above, and the sail is 18 ft tall, so the leverage on this stud is tremendous (17 to 1 leverage), basically if you put a brick under an 18 ft pole one foot from the end you can easily lift your car. As a test you can extend your sail fully by pull it tight, then give the control line a swift jerk, you will snap that stud off. So unless you have your hull re-enforced (like I do) and have a rear stay line (like I do), then adding large sails to your TI is a recipe for disaster.
I have designed and built 3 or 4 different versions of hydrofoils for my TI, and they all run into the exact same problem. Before getting up on the foils you have the drag from the hull plus the drag from the hydrofoils (double drag) so without enough sail area or more efficient wing sails, you simply can't generate enough speed to get onto your foils, so you end up just dragging them around all day (not worth the effort).
One of my foil designs was something along the lines of what Tom was describing. The foils are 3 ft long and 4 inches wide, they are mounted at a 20 degree angle outward (pointing down and out) The end of the foil is about a 1 1/2 feet past the outside of the AMA (passing under the AMA), when the boat is level just the tips of the foils are in the water. As the boat tips more foil submerges increasing the lift. Also when the boat is tilted the other foil is completely out of the water. I had 4 foils (one mounted to each AKA cross bar. The foils were design to remain in neutral (no lift) until you hit 7mph, at that point the lift overcame the torsion bar and allowed the foil to tilt to 8 degrees attack angle. The foils did work to help keep the boat level, but because of the hull design (displacement hull) I was only able to get to foiling speed a couple times, the rest of the time I was just dragging the foils around all day long trying to get up to 10mph. That was 2 1/2 yrs ago the foils have been sitting in the garage since waiting for my wing sail solution. I've done all my calculations, and with a 100 sq ft wing sail main, and my 33 sq ft wing jib (already completed) I can get the TI up onto the foils (fingers crossed).
The TI is a great boat, and extremely versatile, you can do almost anything with it. But the displacement hull and sail design pretty much excludes anything high speed. If you wanted to be creative with foam and glass, you could build a cap to go over the back of the hull to remove the point, and make it look a little more like the WETA's rear (squared off), so you could plane. At the same time add a transom and swing up stronger and bigger rudder like the one on the Getaway. This would also add more floatation to the rear (mine is nearly always almost under water now). When I'm really moving along presently, the rear of the hull is completely submerged under the wave created by the hull. Basically I think I'm applying massive horsepower with the sails to basically drag the boat thru the water, definitely not the way it should be done.
Hope this helps