Aerodynamically speaking, a shorter crank suggestion usually works under the assumption that a lower front end / cockpit is better.
In my testing, that's very often not the case (like everything of course, higher/lower/etc all depends on the starting place) and as tobukog suggested that going shorter lowered his knee at the top of the pedal stroke - this is possibly one of the main reasons shorter cranks can be bad for aerodynamics... more upper leg in the wind all the time. (another possible reason is that the rider generally goes farther forward - and up - in the saddle, which can also be bad aerodynamically, sometimes at least) On the flip side, going shorter does allow a more open hip angle all other things being equal, and many people produce more power (again, depends on where you start) so it's something to test for sure.