Performance will be much better with the quad, there is no risks at unlocking cores, not so much a disadvantage but to run the unlocked cores could need a little more voltage nothing crazy all well with in specs for the processor but since you will have 2 more cores then before the unlock heat can be an issue but nothing crazy like when you overclock. I always recommend having good cooling no matter you overclocking or not.
Now with thats said you will not be able to unlock the cores if they were disabled because they were actullay faulty so not every Phenom II 550 will be able to become a quad. It is kind of a 50 50 chance you will get one that you can unlock into a quad, but alot have no problems being a triple core.