No, the problem is that not all normals on Volition ships ARE 0,0,1. Take a look at some models, like the Herc II, and you'll see. These model bugs probably got overlooked simply because the model code ignored these normal values, and used 0,0,1.
Now, it doesn't require a _massive_ overhaul of all the ships, but it does require some correction. A python script could fix this inside of fifty lines (for example).
By "gun convergence", do you mean that the normal is skewed slightly inward, but by some fixed amount, or that the normals of all guns are variable, so that their lines of fire converge on the target, regardless of its distance from you?
If the first, I'm not sure what benefit that'd get you, because the skew would have to be so small that it'd hardly be noticable. If the second, you can still go around ignoring all the firepoint normals, like now.
However, I think the original question was about guns that fired in different directions, like backwards and such.