520 Racing kits / chainring changes


I have a few questions. I have seen many ads about "racing 520 chain conversions". These usually come with new chainrings front and back and a "racing" 520 chain.

Don't most large bore bikes come with 530 chains and for good reason? Is not a 520 chain weaker? Is this only for less moving weight at the expense of long term reliability? Seems like a very small performance gain for a loss of reliability. Does anyone do this on the street or racing only?

Also, about the chainrings, what is the stock combo (front/back) and what is the most common change made and why?

I am an engineer so I have some obvious ideas about the possible answers but I want to hear from experience as I have never made any such drivetrain mod to a bike before.