Simon Difford is the one recommending Navy Strength gin for this. If it was strong enough to ensure victory against the Spanish Armada, in the Battle of Trafalgar, and the Battle of Jutland then it's probably great in the Six Cylinder, but I find normal London Dry strength (~94 proof) to be plenty adequate. I'd rather just bump up the volume of base liquor in a Negroni-like drink by a quarter ounce or so to lessen the sweetness a little bit. Good dilution helps, too, especially when serving in coupes with no ice-cubes.