I upgraded to the ST Turbo fins for my i14T because I wanted to sail and they are supposed to provide a better centerboard effect. I found that the speed I obtain pedaling is faster without much difference in effort. If you decide to buy them, I suggest taking a close look at a previous posting (viewtopic.php?f=11&t=10176&hilit=adjusting+turbo+fins
) which talks about Turbo fin adjustments to maximize their efficacy. The key seems to be to keep the tension loose in the fins by adjusting the screw at the top left corner of the fin. Using this technique, you get speed without a lot of extra effort.
I would note that the Turbo Fins seems less resistant than the standard fins in that they are already showing signs of wear & tear after less than one season of intensive use.
As an aside: one upgrade that I feel is very worthwhile is an upgrade to the sailing rudder
which gives MUCH better boat control in all conditions.