ninuh, the increased-meringue mousseline is paired with the Miette’s Tomboy in RHC. It is sweeter than the classic version in the Cake Bible- I made a batch recently, but after tasting it decided to add the additional butter to turn it into the Cake Bible version, as it was just too sweet for me.
On the other hand, if you prefer sweeter frostings, go for it!
A further thing I did to keep the white chocolate mousseline from getting too sweet, was to sort of deconstruct the white chocolate. I added all the ingredients of white chocolate except the sugar- I liked it a lot, I may never go back to plain vanilla mousseline. For a Cake Bible mousseline batch, I added 1 oz melted and cooled cocoa butter, 1.5 tsp vanilla, and a little milk, probably about a tablespoon. Not sure that the milk added much, but the cocoa butter and vanilla were great.
A number of the online shops that sell specialty chocolate also sell cocoa butter, I get mine from King Arthur but lepicerie.com and chocosphere also sell it.