My experience with air-to-air gunnery is that it is more difficult than the movies would have you believe and that bullet dispersion can actually help with Ph. Even trying to hit a non-maneuvering towed target is not that easy. I, along with several of my squadron mates, have fired at both towed “darts” and towed “banner” targets that our Ph was not very good. Gun training sorties didn’t happen very often and this contributed to our lack of proficiency. I would be curious how often fighter pilots get gun air-to-air training sorties these days. In some live fire meets, where the pilots send more training time preparing, there was still a desire to “spread out” the bullet to the point that some pilots actually yawed the aircraft while firing.
Now trying to keep the gun sight on a fast jet that is maneuvering for a long enough period of time to get hits is VERY difficult. So from my perspective I would want a fast bullet, but I would put my priority on a gun that has a high rate of fire for air-to-air gunnery.