I see your point about a lay down collar and bow tie but I've tried a waistcoat on with my Balmoral and haven't been a fan of the look so far. If you find a waistcoat that works, I'd love to see photos.

I agree with the A shape of the Sherriffmuir can accentuate a portly build. The Regulation doublet has peaked lapels which create a V shape for the wearer, so that's worth considering.

In any case, I really like my Balmoral and so I can't question your taste. If you like the look, wear it in good health!