A 520 IS narrower than a 530 where it engages the sprocket tooth. Sprockets are narrower, too. So, the load bearing surface is reduced on a per tooth basis. Given the use of the same materials I would say the 530 is stronger than the 520. But, if you made a 520 chain out of say, titanium, it would probably be stronger than a 530 chain made out of ordinary steel. So, build materials matter.
I thought this link was interesting:
http://www.gizmology.net/sprockets.htmFrom some cursory net shopping, it appears you can get 530 chains that range in specs. from 7000 to 11000 psi tensile strength, and 520 chains that range from 5500 to 8500 psi tensile strength.
Here's one even higher:
520RXO: GP Quality Sportbike Chain Tensile strength: 8800 lbs
My guess is that Honda chose the 530 because about 7000 psi tensile strength was deemed as the minimum needed for street reliability. However, if you switch to 520 and then choose a high quality chain $$$, you'll probably be fine as far as tensile strength.
However, the cheapest 520 chain will not be as strong as the cheapest 530 chain. And, with the 520s narrow sprocket width, I would expect the sprockets to wear faster than the wider 530 sprockets.
Cheers,