The BT 21 is a sport touring tyre with harderd ruber in the middle and soft on the sides
The BT 15 is a Sports Tyre, great ruber alll over the tyre
I already use a BT 21 in my FJR, it was a good tyre but only after warming up.
When you have a nice feedback of the BT 21 in the busa please sent some report to us