Well, I think this power could be reasonably compared to the half orc power. Which lets you add a [w] on an attack, that has hit, as an encounter. Now, this lets you be mobile, but you can still miss... Here's what I think (admitably, rule balancing is not my forte):
1) remove the -2 to attack roll
2) make it half your base movement, not speed
3) make it an encounter
Basicaly, its a neat abbility and very "goblin", but it's too powerful to be an at will. If this doesn't seem like its powerfull enough to be an encounter, you could consider making it an reaction to being attacked (in melee, whether on being attacked/hit/miseed is up for debate)
