stock spring is 5.0kg/mm. I'm about 177#. My riding buddy is prolly close to 12-15lbs lighter than I am. The guy that tuned his suspension (richard wilson, house of horsepower) put in a 4.8 rear spring.
It's real real nice! I could see it being a bit weeny under severe circumstances, but for 95% of the riding we do, it's great! (keep in mind he's a bit lighter)
It's not an aftermarket spring..but something off a yahama of some make/model/year. That I don't recall WHICH makes that a useless bit of information.
Still, at 175#, you'd find a 4.8 to be a great plush ride.