Here is a thread on my boat (the ultimate tandem island), it's kind of long but hopefully you can glean some information from it that might help you. ( viewtopic.php?f=69&t=33720&start=60
The AI and TI have many common parts and mainly the only difference is the TI is a little bigger and has an extra seat.
Most of my mods were done because we typically only have light winds in this area ten months of the year, and I don't know about anyone else but it is just plain boring trying to sail a TI in 5mph winds. Most of the guys in Hawaii and Australia, and the Islands have much better winds, and have no need for any extra sails. In my experience if your average winds are 12-18 mph, just the mainsail alone is almost too much sail for an Island and typically needs to be furled in some.
In my experience adding a jib only adds a couple mph to your top speed typically, but does allow you to sail much closer to the wind, (and faster upwind). I don't know if you noticed it but you may have discovered that it is fairly difficult to tack an AI or TI without an assist from your mirage drives, as they tend to be able to sail around 45 degrees up wind (closer if your pedaling), and getting around to the other tack can be quite tricky without going into irons. The jib changes all that, with my current setup, I can easily sail upwind 20-25 degrees off the wind, in 9-10 mph winds at around 7mph (measured by GPS). My TI tends to want to round up into the wind, so I typically need to pedal lightly to maintain that tack though, otherwise I round up and stop. Probably something to do with the wing jib which is a little different design than a normal jib sails and works on different principles I guess.
Hope this helps you