A short time ago I replaced the chain and both sprockets on my Z900. At 5000 miles, they were only slightly worn - my main reason was to achieve a more long-legged feel to the gearing, rather than buzzing around everywhere with more revs than necessary.
I bought all JT stuff:- 16T rubber-cushioned front JTF1537RB-16 , 41T steel rear JTR148941, and a silver Z3 chain JTC525Z3NN114RL.
Everything fitted nicely and the chain runs with no high or low tension areas. Very impressive.
The gear ratios are now more to my liking. I can set off in 1st without having to hurriedly tread into second before the revs (or front wheel) rise too much. All the gears are now more useable and I feel I have 6 useable gears for different circumstances, rather than dancing around in the top two or three gears most of the time. This better suits my style of riding. There is so much torque and power available from the Z that it still pulls the higher gearing easily. In fact it launches and continues to accelerate more seamlessly, since less upshifts are needed, as Jasper rightly said in another thread.
3rd gear now gets 60mph @ 5500rpm (was 6400)
4th gear now gets 60mph @ 4750rpm (was 5600)
6th gear now gets 70mph @ 4100rpm (was 4750)
I have a 17T front sprocket in my toolbox which I might try later, but for now I am well pleased with 16/41.
p.s. speeds/revs are via sat nav. The Z speedo now reads about 6% fast.