Peak power is supposed to only be reached at 6500rpm, so in theory to get the best out of the ST, you should only change at 6500!!
I have personally only taken it up to 6000rpm and I don't do that too often! If I am in the mood for some fast driving, I will take it to 5000 - 5500 revs for the fun of it.
To be honest, it is better to watch your revs rather than speed, as this is a much better indicator to how you are treating the engine. Of course, make sure you keep an eye on your speed .... I'm sure sasst is quoting his/her speed in kph and not mph??!!!