I think we are getting somewhat off topic here talking about comparing different sized Gauss. Going back to the original point of Gauss vs Rails, it is clear that yes, Gauss have improved in viability compared to VB6. I do think that the pro-Gauss lobby as it were is still missing some points. For one, your estimation of costs Jorgen is based upon using a maintenance station costing 1750 BP. If ground based facilities are used, the cost is only 480 BP. Therefore total costs change to 1230BP for the rails vs 2130 BP, meaning a cost increase of 185% to build. What's more the maintenance cost is even more in Gauss' favour. This means they have to be stationed at a population sufficient to man the facilities, but I think that's a fine compromise in many cases.
What's more, railgun tech is basically free, the BFC's are a lot cheaper to research etc. Then, when used on fast ships as PD for a beam fleet perhaps, the Railguns have the advantage that they can be used more easily as duel role offensive and defensive armament. So yes, certainly Railguns aren't as dominant as people were claiming they were all the way to fairly end game techs, but they do very much hold advantages well into Gaus ROF 4, 5 and potentially 6, and the choice of Gauss vs Railguns is probably going to depend on what your primary striking weapons and fleet doctrine are.