I did a 520 chain on my ZX-7R my buddy is running a 520 on his GSX-R 1000 so even though the box for the chain says 750cc max I think it will handle the 900cc motor, btw he and I are running RK X-ring racing chains. I am running rethenal sprockets I was down one up front and up 2 in the rear and WOW it was hard to keep the front end down. Id say depending on what your looking for stay with one down up front thats like the universal works with everything sproket change because it helps acceleration without taking off too much top speed. When I changed my sptockets I went from being able to burry my speedo at 180 MPH to just being able to hit about 155 MPH indicated. Now I got there a lot faster than I used to infact I could beat my buddies 2001 R1 but he took me on the top end. The only way Id do 2 up in the back is if you were gonna be racing on a tight course and needed the extra acceleration or if you wanna rip some sick wheelies in 3rd gear. For the street though 1 down is fine, I made the mistake of trying to make my bike as fast as possible on the street and I got into wheelies and all that and ended up spending 2 weeks inthe hospital when I crashed, the ZX9 is plenty fast, Id just get new Rethenal sprockets and an RK 520 chain all together should cost about $180 remember though you have to replace both sprockets. Hope this helps.