true, true- but on most old 5-speed gearboxes, 4th gear war direct anyway, so actually only 1st, 2nd and 3rd would have seen prop shaft speeds below engine speed. the thing to consider is, with the current setup, the prop shaft will be under less load at high-speed, due to the fact that while it has to transmit the same huge twisting force required to push the car through the air at speed, it won't have as much centrifugal force acting on it at the same time
the only draw back with this sort of setup with a normal gearbox, is that the synchros essentially have to "brake" the speed of the propshaft as well as the reciprocating mass of the engine before selecting the next higher gear. but due to the fact that it's made from carbon/kevlar and no doubt a lot lighter than steel, and that the DSG gear pre-selection eliminates most of the synchro wear, all is well.