One of the more interesting kit bashes I have done was the conversion of a Scharnhorst with 9x11" to 9x13.8". With the original I left the 5.9" and 4.1" of the original armament.
With a lot of these DP conversions, I will put the 5" where previous weapons have been as this means there will be ammunition handling facilities in the same place.
A dual purpose Scharnhorst with 13.8" would be a formidable opponent for the British Battleships and battlecruisers of the time. The saving made by having the DP armament over the mixed secondary/tertiary guns, put into extra armour would also help. The standard Scharnhorst with 11" is 785 feet, while the 13.8" version is 802 feet. The 11" weighs in at 31,800 tons standard, the 13.8" at 36,000 tons standard (more breadth, more armour).