I recently upgraded my drive in my 2016 outback with a 180 Drive with turbo fins, my previous drive was a V2 with turbo fins. I noticed a slight increase in speed as in my first kayak fishing tournament this year in florida i had documented speeds up to 5.2 mph in pretty ideal conditions with a relatively light amount of gear. I would say the 180 drive gave me about a full 1 mph increase in top speed. I obviously understand that doing this to the drive is like driving a car at high rpms not really good for it but if you need to its ok.
