The stealth detection increase is +30, which is the same as 6 levels. This means a level 64 hunter can detect a level 70 rogue at the same distance as a level 70 that doesn't have stealth detection increasing effects.
This is immensely useful in PvP. In a battleground, if you hear that a Rogue is nearby turn on Track Hidden to expose it as quickly as possible.
This only increases your stealth detection and places the mobs YOU DETECT on your minimap. If a mob is much higher level than you and stealthed, it is likely you will fail to detect the mob before the mob has a chance to attack you.