I did this and here is what I found. It was easy to do and I did fit a speedohealer in the end. It really did transform the bike and had no effect on fuel economy. Sprocket came from Wemoto. There is a thread or two about this on here somewhere
Don't know why Suzuki didn't do it as standard. 6th is more useable and much better drive out of corners and islands in 2nd and 3rd. Very pleased. Speedo error according to SatNav averages 12% when checked at 30,40,50,60,70 and 80mph. According to my simple maths that's 6 and a bit percent from the gearing change an 5 and a bit percent built in by Suzuki. If you use 10% for mental arithmatic it's near enough and easy to do at a glance when passing cash cameras (but still might get a speedohealer!)