My brand new (well 2.5 month old bike) was destroyed last month in an accident. That bike had a compact double on it (Campy Record, if that matters). I picked up the replacement bike at the shop on Saturday. Because of a knee injury from the accident, I went to a triple this time (still Campy Record). It required a different front derailer, a different rear derailer that was a little longer since the chain would have a larger range of motion to deal with, and a longer chain. So, except for the brakes and levers, everything else was different. It did add .6 pounds to the weight of the bike (everything else is identical to the old bike). I would expect that weight difference to be bigger with other component groups, since Record is about the lightest out there.
That all I know on the subject,
Susie
"It never gets easier, you just go faster." -- Greg LeMond