Right. After 3k miles, my 999S needs a new back tyre. The OE was Michelin Sport Cups, with a 190/50 at the back. I am opting for a 180/55 to replace it and will report back with my findings concerning moving down from a 190 to a 180.
I still have a lot of wear left on the front tyre, so I'm staying with Mich for now (as I've no complaints) but I reckon I will replace front and back with Bridgestone BT014s next time, to see how they compare.
Decisions, decisions. Wish I had a rodent to advise me, like Jools has.