Lighter wheels allow the bike to handle quicker. My SP has them and while I can't say whether it's due to the wheels or not, the thing handles so quickly it almost feels like it's defying physics. At the same time, super light wheels take away a little stability that you might get with heavier wheels.
In general forged wheels are better because they're less likely to crack, more likely to bend, and stronger so they can use less material and get them lighter.
I don't think you will really get a performance benefit unless you're competing at a high level.