I just came from a mission wth this sub, and I have tested this feature.
Man, the speed is really great. Very usefull when you want to sneak up to a ship at periscope depth. It is very usefull to be faster then normal ships when you are submerged. I made good use of it when i wanted to hit on a smal convoy. Getting in front of them (actually passing them underneath them), repositioned to get my bow to them, and fired some fish, just to get them running into them
Speed is really the key to succes.