Or, are we really talking about a frame with nice thinwall tubing like the RB1 built with a fully modern group such as Campy Veloce (or better) 10 speed or modern Ultegra? It that case I'd expect the RB-1 to hold pretty near even with modern steel bikes of similar tubing and geometry.

No good reason other that upgrade cost why a bike on a vintage frame needs to be less nice to ride than similar build on a modern frame.