Here is a great discussion on this very topic: http://www.600riders.com/forum/fz6-general-discussion/32068-520-chain-benefits.html
Basically, the 520 chain offers weight reduction in the best way possible (Unsprung weight & rotational mass), yielding quicker acceleration.
Yes, this set-up is "weaker" than the stock 530 chain configuration but it will still offer years of reliable use with proper maintenance.
Hope this helps!
