It seems obvious, but missing from the comments, btl-a4 ion allows for repeated ion, and the primary attack, which means, before, the b-wing that required 8 ion hits to destroy, requires far fewer hits, because you can land the additional damage.
It also means, you're effectively adding an additional ship for the cost of the turret. Just pretend that ship is overlapping the y-wing exactly. 2 z95-s cost 24, for two two attack ships, a single y wing with blaster cannon and btl a4 is 22 points. Gives us enough room for an astromech, or bomb. We already know 3 dice give expected damage similar to 2+ focus. It's giving up the action on the first shot that seems not as great, but often our bandits spent that focus on defense, which the extra hull/shield of the y wing sorta gives us a defensive boost anyway., so the offensive profile is about the same.
What I find interesting is the number of people talking about full squads of these. I think it's the one or two mixed in a squad that's going to be very good. Super friends will love this... Dutch + r2d6 + predator and a blaster cannon + a4 + Garven/Kyle ought to do well.