Well when I was skinning the new C.205 it originally only had the cannon option as well. I wasn't sure what the current version had so I had a look at AH Wiki. This showed both options for the C.205, so I assumed that was already the case in-game.
I mentioned to HTC that the four mg version might be useful for Sicily and early Italian campaign scenarios and they later added it to the new C.205. I guess the lower weight should marginally improve performance but I can't see it being a popular choice in the MA somehow.....