The reason for the 520 conversion is just a Dank stated prior...basically the sprockets and the chain weigh less, so you have less rotating mass. Which equals better handling and a few extra ponies, due to the lighter weight.
If you are riding it on the street it is going to totally be your preference, I prefer the 520, but then again I race mine on the track, so every little bit helps.
One thing to pay attention to is the tensile strength of the chain, you'll find that a high quality 520, like a DID ERV2, will have the same tensile strenghth as the average 530.
Either way you'll be fine.