A couple of thoughts on this..
What will work 'best' will ultimately depend on two things. The first is the handling of the ship. If the handling is crisp and quick enough to track smaller fighters, then Gatlings (or other repeaters) could be the best choice as they do superior damage over time. If the handling is NOT fast enough to track a close fighter (Freelancers barely get there) then Cannons would be indicated, as they can deliver a lot of damage as you make a crossing/timed shot. Reading into this, the target is as much a part of the equation as the MSR.
An example - I fly my 315p with fixed GT-220's. Against small stuff like Bucc's, there are misses, but the hits quickly eat them. For anything larger than a single seat fighter it becomes a no-brainer as that ship has no problem keeping it's guns on anything even slightly larger than itself.
The second is controls/playstyle. This one is harder to quantify on a DPS table, and really comes down to how well YOU as the pilot can control the ship+weapons combination. Fixed work well but take quite a bit of practice.. gimbals (even since the last balance pass) do an excellent job of making up that error between your ship position and the target. They can also allow a slower handling ship to apply much more pain to a smaller one. Anyone who doubts that has never been on the business end of a MIS. (ANY Freelancer. Stay out the front!!)
The MSR looks to have 2 size 3 hardpoints under pilot control. For the size of ship this is not a lot of power to project, so aggressive piloting will probably be the answer, and a reliance on crew. The two turrets come in underwhelming as well; CIG has not positioned this as a combat ship.
However - with a crew, or eventually NPC's/blades, there isn't anything small to medium that can stand up long to those six guns pouring fire. And in concept the coverage for those turrets is superb.. there do not appear to be any blind spots in the design.
I am expecting this ship to become my new daily driver so have put a lot of thought into how it will need to fight and thus the length of this post. Component choices and handling are the two areas that are still grey, so we will see.
@Black Sunder None of this is stuff you don't already know, but hopefully others will find the thought process useful.