Had a similiar issue with going off the end of the tread without much effort with the BT21 and after a trip to Donington and a few hairy moments took them off and Focced them in the bin. Never used bridgestones again.
I find the PR3 good plus they last well, but for handling and confidence I find the Metezler M5 front and rear brillant.
My son had BT21s on a K5 Gisxer600 and was constantly using all the tread. He put M5 on and never had a problem either. Never got to the edge on the M5s
BT 21s were fine grip wise, but were slower to warm up than the PR3 and the M5 and I was not to happy about going to the edge of the tyre to easily.
I find the PR3 good plus they last well, but for handling and confidence I find the Metezler M5 front and rear brillant.
My son had BT21s on a K5 Gisxer600 and was constantly using all the tread. He put M5 on and never had a problem either. Never got to the edge on the M5s
BT 21s were fine grip wise, but were slower to warm up than the PR3 and the M5 and I was not to happy about going to the edge of the tyre to easily.