i had a 525 sigma on my 998R and will putting the same on my 999R.
Keeping standard gearing as it pulls 10.5k in top easily everytime i ride it.
Change my chain and sprockets now at around 7.5k as i've had chain's fail on:
1) Fireblade RRV (14k) demolished the engine, needed replacing.
2) GSXR1000 (9k) no damage luckily.
3) 998R (8.5k) no damage luckily.
If you ride hard/wheelie/or have a lot of ft/lb to play with i wouldn't use a 520, you won't notice the reduction in gyroscopic forces etc.
Peace of mind with good chain any day