IMO, the Geb would be good as a battlecruiser. The Geb wasn't exactly in the OP, but for what it's worth, I agree with Epo's statement.
The Osiris can indeed fire 5 primaries forward - however, I think you have to have the ship at a slight angle in order to pull that off. I wouldn't be opposed to having that 5th primary's firing arc restricted to cover the aft sector of the vessel.
To Lythrilux: I am attempting to address both RP and balance issues. The Marduk would still reign supreme, but the Order and the Core would be able to even the odds a bit with more robust BSes. Is this shallow? If it is, what would be your remedy?