Join Date: Jan 2007
Location: Sacramento, CA USA 96814
For most 600 class sportbikes many people go -1 in the front and +2 in the back.
Advantages - more low end torque.
Disadvantages - decreased mileage, and inaccurate speedometer and odometer readings (they will read high in proportion to the gearing change.) So yes, you would want a speedohealer.
If most of your riding is in town, or in aggressive canyons, or you track the bike, it's probably worthwhile. If most of your riding is on the freeway for long trips, it may be annoying.
Also, while the 520 has plenty of strength for a 600 cc application, it won't last as long as a 525, so you will replace chains and sprockets more frequently.