The argument that the fixed Ret should cost only 345 pts confounds me somewhat. I get the feeling that the only reason it's being presented is because the current Ret is 345 pts and weak, and therefore there is resistance to the idea of any form of cost bump.
However, let's look at the original costs. The Emp was only 345 pts and the Ret was 365 pts. Now, this was obviously unbalanced, but I put it to you (the BFG community) that if the that original Ret was 18WB at 45cm instead of 12WB at 60cm then not only would people have taken the ship but most likely there would never have been any change in the cost of the Emperor. I believe the reason for the points swap was because people complained about the terrible disparity between the Emp and Ret and the HA thought it more a problem with the Emp than the Ret and so figured the swap would fix all problems (of course, they were wrong).
The point here being that if the HA had plumped for the fix to the Rets broadside firepower that we are talking about it would have been at 365 pts, and the Emp still at 345 pts. I imagine that most people would consider this situation slightly unbalanced still, but it is certainly better than the current (official) state of affairs and an acceptable arrangement.
That is an argument from tolerance though, not balance. When looking to balance then we should look at what we have, which is 12WB at 60cm for 345 pts. While this arrangement is suboptimal to the role of the ship I think it would be hard to argue that this version of the ship should cost less than 345 pts. That is, it's balanced for what it gives, though not particularly attractive. So, starting from this point, we swap 15cm range for +50% firepower. This is not equal. The firepower is worth quite a bit more than the range. Add to this the fact that the ship is now optimised to its role and a 10 pt increase really is quite conservative.