I've just purchased a bike with a 105 group-set, 50 + 34 chainrings and a 11-34 cassette.
I was expecting to be able to replace the cassette with a closer ratio one, with a slightly lower top gear: maybe 13- or 14-. However the shop I purchased it from said the 105 rear mech wouldn't accommodate cassettes with a top ring smaller than 11 or 12.
Is this accurate? Surely if the derailleur can handle the combination of 50T chainring and a 13T or 14T sprocket two or three gears down from top, it can handle it 50T combined with 13T or 14T as top gear as well? Wouldn't a 14T-32T cassette require less forward and backward take-up from the derailleur, compared to the 11-34 cassette that's currently on the bike?