It's realy good
I have the "TT" 21 inch front with a Saxxes rim on the 1200 GS Hub. The differences I've found so far are.
1. Surprisingly better in loose gravel & sand ( a lot more planted)
2. No real difference in Road manners until you really start pushing hard (dragging the pegs, 180 Kmh + etc)
3. You lose the last 5 % braking before Lockup
It makes it easy to forget the 300 KG until old Mrs physics catches up & it's time for a wee lie down ( after sliding)