So my point is even more valid if roadies are going that much faster than MTBers. 140 road hydro discs are too small
EDIT: too slow! (pardon the pun)
40mph is a very conservative top speed for descending on a road bike. I live in South London and can hit 50 no probs on a few roads near here and have to slow to zero for junction etc at the bottom. These speeds can be reached on local road race circuits too so the example given by mike d is relevant.