I intuitively believe that it's primarily the components (ie brifters, 10 cog rears, aero rims) that make these modern bikes marginally faster. Whatever the difference four pounds less weight of frame would make for a professional racer going all out in competition, I am quite sure that the gain in average speed that would separate the steelies from the plastic bikes would not be as big as the difference that separates the Tour de France champs from the mid level riders. In other words, if you took an Armstrong or a Cantador and forced him to ride steel with brifters he would still win and still beat the others by a good margin. This is unfortunately a scenario we will never get to see. Trek wouldn't make the bikes out of steel even if they were faster. A carbon fiber miracle bike might be a little faster but what is fastest about them is how quickly they can be duplicated, literally flung from the gloomy depths of the slave labor factories of Asia, each one a perfect clone of the one before.... each gleaming example of modernism ready for a year or two of hard service before making the ultimate ride to the garbage dumpster.
Garth Libre in Miami Fl. USA