IMHO, if you want to increase the performance of your Island (TI or AI), add a head stay. Don't use spectra line, but a line with a bit of stretch in it (nylon based?). Lead it through a block at the bow to a cleat aft, so it can be released for furling. And add tramps so you can get out to windward.
Consider: Hobie designed the bendy mast for the safety of the majority. A good gust, the mast bends, spilling wind from the top of the sail. Safety feature. A head stay will limit that bending moment, keeping the top of the existing sail working in stronger winds.
Also consider a downhaul on the mainsheet for close-hauled sailing. This also helps keep the top of the sail at a closer angle of attack.
Although I've only had my TI for ~2 months, my planned speed improvements, in order, are:
1. Get a crew(person) out on the windward tramp.
Speed Increase: Substantial, I think, by a
long shot.
Cost: Nothing: requires NO mods to your Island (assuming you already have tramps).
Complexity: Minimal. Just need an agile supermodel.
2. Add a head stay. May be more effective in the TI, as it has an attachment strap at ~4/5 mast height for the 'future jib'. On the AI, I'm not sure how the mast will bend in between. Might even be unfavourable for the AI.
Speed Increase: Noticeable, I think.
Cost: low: just a pulley, cleat and line.
Complexity: Low. Even a supermodel can yank a line...
3. Add a downhaul. When close-hauled, the mainsheet also works as an outhaul. However, sail shape (especially at the top) could be improved using a downhaul. Given the relatively narrow width of the main hull, it cannot be placed in an optimal position, and also has to be able to be swapped from one side to the other.
Speed Increase: Noticeable, I think.
Cost: low-medium: fairleads and cleats on both port and starboard sides, S hooks, and line.
Complexity: medium-high. Whilst two extra lines in the cockpit rate a medium, having to manually swap them between tacks rates a high. The supermodel just quit.
4. Add a furling jib. This is a subject in it's own right, so I'll start another thread for that one.
The supermodel is following to the next thread, coz it sounds vaguely cool
Disclaimer: I'm just an aero-hydrodynamics theory enthusiast, who by now has had a few Scotches. And for the ladies on this forum, my wonderful wife Sandy just gave me a smack on the head for being chauvinistic. My apologies to the supermodel.