Hmmm, you are not going to run out of tyre on a GS (by that I mean run off the edge of the tyre). Ground clearance and talent yes but not tyres. Fit what is recommended on here. Just one thing to take into consideration. PR4/5 have sipes, these things helps not only with water dispersal but also heats the tyre up more quickly. The movement this creates might be unsettling if you can feel it through the bars and seat but have faith, you have lots of traction. Otherwise get a tyre with less grooves or without sipes and preferably no grooves on the edge. If it is going to be a wet day, PR4/5 every time.
Somebody will be along shortly to tell me I'm talking bollocks. In that case please enlighten the readers of this thread.