First congrats on your purchase you will love your AI, especially as you get more used to its unique capabilities. Though I don't own an AI I am very familiar with the design, and sail the two seated version of the same boat (called the tandem island), they are virtually the same boat except the number of seats.
There are many AI owners who are extremely happy with the sailing performance of the AI as is out of the factory and feel no need to want for more. Keep in mind the boat is a pedal boat with the mirage drives primarily with the sails to assist as needed (don't lose track of this concept, as the boat will never be a sunfish or laser (it's way more than that). The sail is fully furl able so depending on conditions you can dial in as much power as need to suit the conditions. Yes the boat is lacking in performance in low winds (below about 6-7 mph wind) but when winds are light you can supliment the power by pedaling (can't do that with any other boat). As soon as the winds get over 10 mph then the AI comes into it's own and the standard sail provides more than enough power to propel the boat, actually if it gets over 15 mph wind you need to typically furl the sail in a couple turns or it overpowers the boat, so putting on a much larger sail is probably not a good idea as the floatation on the AMA's is pretty evenly matched to the standard 60 sq ft sail adding more sail just results in capsize, which believe it or not is very difficult to do with an AI (a huge plus).
I suggest spending more time with the boat as is before making any mods, I think once you gain some experience you will be quite satisfied with the stock setup.
As far as retrofitting the new style rudder onto this AI I recommend against it, the standard rudder was always sufficient on the AI's, it was the TI's that had the rudder issues, later on because the AI and TI are sisters, the AI got the same rudder as the TI, but I think this was just for Hobies manufacturing purposes more so that the AI needing it (most AI users were very happy with the old t/s rudders, and many continue to prefer them.
Definitely get the turbo fin upgrade kit from Hobie you will be happy you did, as long as your at it you might look into the upgrade to convert the drive to a v2 drive vs a v1 drive (the v2 drives are a little better IMO). Even if it's a couple hundred bucks, your getting virtually a new $700 mirage drive for two hundred bucks (most wearable components are replaced with new as part of the upgrade (really worth while in my opinion). I'm curious though as the stock AI back then came with the turbo drive I thought, maybe someone pulled a switcheroo or you already have the turbo drive.
Hopefully some of the AI guys will chime in and give you some real advise.