If you liked the 21 you'll love the 23. As Skydivr says, the BT016 front gives you great grip and performance for the control point and the dual compound BT023 rear will add many miles while giving plenty of grip for knee dragging on the twisties. This is the combination I use on my busa and it serves me quite well.
However, the new BT016 Pro which I have no experience with, is advertised as an improvement in mileage as well as performance over the standard 16. You may want to give it a shot if you are more performance oriented.
You cannot go wrong with either tire or combination there of!