I researched this a couple weeks ago, as I came from a drive-shaft bike, and had no idea of what these chain numbers meant.
The 5 part just describes the distance between the centers of the link pins (or wherever on the links, basically how long a link is) and the next two numbers describe the width of the chain. So a 520 chain is going to be narrower than a 525(and thus require a narrower front and rear sprocket) and lighter...this is not necessarily a good thing. Wear and durability will be affected, so you can expect your chain to not last as long, especially with the copious amount of torque that our bikes produce, all to save a few grams of rotating mass. It's up to you to decide if you can tolerate a shorter lived chain to gain performance. When I was researching it, I found other sites (the most helpful was in reference to trick riding Japanese bikes) that basically said, don't go under a 525 if you have over 1000cc--remember these guys are pretty much only doing wheelies and stuff like that, so keep that in consideration...their chains must be stronger to withstand what they do.
A 530 chain is wider still than a 525, so it is going to weigh more, last longer, etc...so far I haven't seen any even available for our bikes, so this is a mute point.
One big point to consider though is: Do you want to replace the front and rear sprockets? if not, a 520 chain is out of the question. if so...you will have to do it all at once...no incremental changes like going to a 14T front sprocket instead of stock 15T at a later date.
There's also the risk of the chain actually failing.
There are a few race guys that use a 520 on the big bikes to save weight, reduce rotating mass and rolling resistance.
But, the assumption is that they change the chain more often to not let it weaken to the point of failure.
I would not go to a 520 on a high torque, high HP bike like the SF or a 1098/1198.
The added weight savings and smoother rotation is not enough to risk it.
IMO a good quality 525 is the right chain for the Streetfighter.
Oh and Remo, That is not a stupid question - it's a good one!