It looks like the better Aero of the Silvio makes the difference.
On the Cycle Oregon 2013 I was rolling down hills 3 to 6 kph (2 to 4 mph) faster than Bacchetta Giro 26" and Corsa 650 with 70 to 90 Kgs riders, compared to my 106 kg light weight.
I asked a few that had mountain bike shoes, if they wanted to swap bikes on the decent, to see what difference my weight made rolling only, but none would swap!!!
I suppose they did not trust me, as on previous descents I was passing them 15 to 20 kph (6 to 12 MPH) faster when pedaling hard