Really? Bison is not in any different league. Its a 60s airframe with avionics upgrade. Very similar to Romanian Mig-21 "Lancer" upgrades. In terms of maneuverability, the F-7PG would run circles around the Bison. Avionics wise too, there is only so much you can put into the nose of the Fishbed. Even if you make the Mig-21 BVR capable, it would have a limited range owing to the small size of the nose cone mounted radar (Kopyo is no exception). Also currently, the BVR capability is limited to semi-active which has its drawbacks and can be countered by ECM and traditional countermeasures (expect performance to be around that of the AIM-7 sparrow).
Pakistani F-7PGs have Italian Fiar radars with very good range (out to 40nm) which can be used in BVR engagements. Currently we do not have a BVR capability integrated on it. However even if it is, the engagement envelope would be limited.
On the topic of BG vs PG, I think both are about the same. One thing to note is that the BG radar is even more limited in range than the Fiar-7 radar on the PAF PGs. So eventhough BVR AAMs would be integrated, some enhancements may need to be done to increase the range of the radar or use an active BVRAAM. I am sure BAF will do that in the near future.