I remember when I got my Roscoe 7 string and discovered that there's such a thing as too wide. 7 strings at 18.3mm is way too wide for my hands to play. I had the added problem of not being able to lay it down horizontally. Planes would keep trying to land on it! Anyway, what's your ideal bridge spacing for extended range basses? Maybe the manufacturers will take notice. I'm voting 16mm myself.