Personally, I've never understood the urge to swap a 525 chain for a 520.
If weight savings is your goal then you're far better off either losing a bit of weight yourself (the cheapest mod there is) or going for lighter parts elsewhere on the bike.
A broken engine case from a chain failure at speed isn't something you'd wish for and the slight weight savings going from 525 to 520 certainly don't justify such a risk.
That's just my thoughts on the subject.
You do whatever you feel like but keep in mind a 999(S/R) produces quite a bit of torque and at some stage it might exceed the tensile strength of a 520 chain.