Old thread, but still fun, I used to have a sunfish and loved it (brings back memories).
If you thinking about adding sail area to a TI there are only a couple areas that you need to pay attention to.
First off, the mast itself is very strong and shouldn't give you any problems adding more sail (within reason of course, and be wise about using it). The V brace system for the mast on the TI (and AI) is very strong from side to side and you shouldn't have any difficulty with that either as long as you check it often and make sure everything stays tight.
The problem when adding sails is in specifically 2 areas:
The bow on the TI is fairly weak and if hanging a sail on the front you need something to attach the sail to. I'm not sure I would trust the lifting padeye for too long (an accident waiting to happen). And with a sail on the bow the bow moves around quite a bit and your hatch will leak much more. To fix these problems I made a V brace for my bow out of aluminum that is clamped to the ends of the front AKA crossbar (strongest point on the boat), then form a V in the front and attached to the padeye. The sail is mounted to the V brace, this fixes two potential problems with one piece.
The other issue is if you look at the design of the mast holder, you will see it is braced very well side to side (you shouldn't have any problems there), and the front AKA cross bar (with the white bearings) on newer boats is double welded, so that will never give way. The problem lies on the bottom of the hull, there is a 1/4 in stud that holds the bottom of the mast cup to the bottom of the hull. Of course the mast cup sits on a beveled riser molded into the bottom of the hull, but honestly there is very little preventing the mast from pushing forward or backward given the 17 to 1 leverage of the mast on that joint. Basically if you took an 18 ft pole (length of mast) and placed a brick 1 ft from the end, you can lift your car with the short end. This analogy outlines the stress on that tiny 1/4 inch stud. On my hull I cut a small piece of aluminum about 1/8 x 2" x 8" and placed it behind the mast holder and up against the mirage drive pocket. I then built a clay dam around it and buried it in epoxy (inside the hull). Didn't take more than an hour, but now I know the hull bottom and mast bucket are strong enough for heavy duty sailing, and able to withstand extra sail area. (which I do often).
If adding just a jib you probably don't need a rear stay line, but if you are planning to add a huge spinnaker (like mine) then I highly recommend adding a rear stay line. The forward force from the big spinnaker is huge, and can damage your boat without a rear stay ( I snapped a rear stay line once, and that 3/16 line was at least 300 lb test) I now use heavier line that is stretchy (nylon).
All my mods are well described in the thread The ultimate Tandem Island ( viewtopic.php?f=69&t=33720
) in there you will find designs for mast toppers, re-enforcements for this and that, and all the crazy kinds of things I've done to my TI's (tinkering is part of my hobby), hopefully you can find something in there you can use.