Rather than a ship mounting several 155's, would a ship mounting a naval rail gun not be a better choice of armament. I mean they are in testing phases now, and yes, they will need a bunch of hardware developments before being viable, but still in the next 20-30 years I wouldn't be surprised if they became the main mounting on ships (well... maybe not all. A normal gun can do some things a rail gun couldn't)
I don't really see rail guns becoming the main gun mount on ships until the late 2040s at the earliest - it's the fact that they have such large power requirements and I do foresee a global reduction in naval procurement over the nest decade or so. These, combined with the fact that a rail gun is going to be expensive to develop independently pushes back the IOC date. This is not to say that the USN won't have them in service sooner, only that the majority of the arsenal will still probably be 5" and 155mm guns.
Lately I have been picturing naval railguns, like however many years down the road, as less like deck guns and more akin to some kind of swiveling box-like "gun". Something like the positioning of the Tomahawk box launchers on the late Iowas but instead of opening and firing missiles they contain all the terminals and they swivel, possibly elevate on the surface like a box launcher. This is going by current railgun technology where you need those giant bolted surfaces to form the corridor for the projectile. Unless they can find a way to get around it, I just can't picture a railgun on a conventional barrel and turret design.