if it goes ahead, there will need to be significant loading gauge increase and probably some divergences from the Origanal alinement. Not to mention Re sleeping Variable gauge sleepers and will it create any gauge orphans, ect. I'm sure we all know that Never-less It shall cost a lot of $$$$$$$$ to do wether you agree with it or not.Which is why it won't go ahead.
Divergences are a separate issue and separate budget. If it was viable or so important if should happen regardless of the rail spacing width.
Conversion of NG to SG is more expensive than might seem as the foundation of the track will likely need to be widened. Will need 1000km of new sleepers and every set of points along the way rebuilt.
And at end of day, how many extra tonnes will this increase capacity, 0t!!!!