Found it -
None.
Or More accurately, treat the bay as a battery of turrets of the same type - so a 50 ton fusion bay at tl12 is a 7 factor fusion battery.
This gives it a +7 BAB.
it would take 16 Fusion guns mounted individually to form the same battery with a higher cost, EP drain and shorter range. The Fusion guns would take less interior space however the bay improves better with tech than the turrets do.
On the whole - if you are big enough to have a bay, you can afford the interior space and aren;t likely to be using it against swarms of fighters, then a bay is a more effective option.
I think of bays as a collection of lots of small weapons - firing as a group
At least that's my interpretation - I expect to be proven wrong, but I figure that an answer that works is better if you need a quick answer.