The engine upgrade early in the game (1500 renown or so) is nifty; that will bring your standard surfaced speed to 16 kts. 20 at flank.
There're always anti-sonar coatings, snorkel, radar and such that need to be bought if you plan on surviving. Remember that a u-boat is passive until it is in battle, whereas the enemy is active until he is attacked. The thing to underline is that you should only allow yourself to be detected when you have already sealed the doom of a few T-3s and maybe some unlucky destroyers.
