IMO, a common misconception about the Turbofins is that they are just for stronger pedalers who can utilize their greater size. While they are larger and have a greater power potential, they are also more efficient and can be advantageous by applying the same effort (at a slower pace).
I took them on a five mile (GPS) cruise this morning at the local lake (no current, light winds, calm water, up and back course and I wasn't sailing). In the Adventure it took 58 minutes for an average speed of just under 5.2 mph; could have gone further at that speed. Normally I don't go nearly this long without resting the old legs, but I figured a good speed over this distance would be due to efficiency rather than power -- wanted to see if it worked out that way.
I don't have any comparable info on what the standard fins would do over that distance, but the best I've done over half the distance was 4.5mph with the standard fins.
I would imagine that any of the Mirage drive kayaks would get similar improvements. Has anyone tried them yet on the Outback, Sport or Classic? The tandem could probably tow a water skier with a pair of Turbodrives!