I think the Nambos have something different. Never worked out what, but its more than the head. Maybe crank case volume?
You also assume you can get the port timing right with the S3 insert. IE my 300 with ports timed where I wanted and the HIGH HIGH insert gave me a minimum clearance of 1.9mm still. Granted 1.3mm of it was gaskets which could be removed, but they would have altered port timing.
The process you described however, is exactly what I did with my S3 250 Cylinder to get 1.25mm squish, then had the head machined back to 14:1UCCR.
I have a gold one, and a red one. Well aware how good they look, but as for being functional/practical.. not so much. $$$ bling!