Created on the fly for a guest a table who asked a server for "something like a Negroni but kind of caustic," and this is what I whipped up. He liked it so much, that another was ordered by someone else at the bar.

Thanks for the back story Fred.  It's been well over a year since I tried this, but your recent comment made me re-visit this drink.  I just whipped this up as a post shift drink using Carpano Antica Formula instead of the Cinzano + vanilla syrup combo (which seemed apropros given CAF is a rich vanilla bomb compared to the Cinzano).  Didn't have Cinzano on hand to do a direct comparison, but it was plenty rounded with the CAF, if anyone is looking for an easy sub.  Might also try this with Abano swapped in for the Averna, been on a Abano kick lately.  And the grapefruit oil / aroma really does tie it all together nicely (a trick I've frequently used myself with stirred Scotch based drinks).

I had some trepidation about combining the Sfumato and the peaty Scotch, but instead of amping up the smoke as I feared, they somehow blended into one another providing a continuum that tied the other components together. The vanilla syrup contributes to rounding this out, but combined with the Averna this tilts sweeter than one might anticipate--I wonder if half a barspoon of vanilla syrup would work?

Tried this with Bowmore Legend. Nice, but a bit sweet, even with an extra 1/2 of Scotch. I'd try 2 oz Scotch and 1/3 oz each of the rest. Not as elegant a ratio, but it would be nice in metric. :)

I made this with Punt e Mes and my Amer Boudtreau (very bitter, high proof). Very good, but a bit too sweet. Suggest reducing Falernum and Amer to 1/4 oz. Maybe try rye.

I would decrease the lemon and increase the Bärenjäger

I agree on increasing the Bärenjäger and decreasing the lemon. I wish the Becherovka stood out more.