What this talent helps with is in-combat speed. People who do not spec sneaky will have their sprint canceled while in combat. Thus, they will be moving at 100% of normal speed, instead of 135%, as is with sprint on. With Sneaky trained you will move at 115% normal speed while in combat. Your sprint is also canceled, but your natural 115% run speed is still active.
Many people confuse this talent as adding 15% to in-stealth speed, which it doesn't do. This is a very useful talent for not only chasing people down running from you, but also running away from folks pounding on you! They will move at in combat speed (regular speed/100%) while you move at 115% speed.