From almost new, I changed to a 15T front and 41T rear sprocket, and have always run 180/55 rear tyre. For me, this gives a very good ride/response. I installed a "Speedo Healer" and this is really easy to install and finely calibrate (against a Sat Nav for example) to give you a perfect speedo reading. Mine is accurate to my Garmin right up to 75mph. Obviously it doesn't correct the mileage reading, so I use a formula to give me mileage reading and actual; the bike mileage shows less than actual.
(UK-spec. SP2, 2002 owned from new, 45k miles)