Its law in most countries that speedos do not underread , so to cover themselves for someone putting on different sized tyres or whatever, all vehicle manufacturers make the speedo overread.
Consensus for the GSX S seems to be 6%, although some of the figures stated here don't tally with that.
I once had a ninja 250 speedo that read 100 when my actual speed was only 83, it took me a while to work out why I always had traffic banked up behind me.
Once you work out what how much the difference is it's not really a big deal, I just mentally add it on (and the police tolerance) to the speed limit and go that fast instead.