[Zero10]

People argue that the extra exhaust duration causes overlap, meaning lost boost. However, I have seen diagrams of the camshaft, and I don't believe there is any overlap. If there is, I sure am not having any boost problems as a result of it, nor any exhaust backfiring (which would be indicitive of this problem).
So, IMO there is no downside. And yes, the purpose is to help the exhaust leave the cylinder faster (higher exhaust velocity = quicker turbo spool) as well as force that little bit more exhaust gas out of the cylinders.