Lemme jump in here....

First, I can easily tell the ride difference between a set of factory-engineered low-spoke count wheels and a set of 32 hole box-rimmed wheels on my single. My Ksyriums have way more spoke tension and ride much harsher than my standard 32 hole wheels. How that would translate to a tandem, I not too sure. As the captain, I don't feel a whole lot of anything.

Now as far as speed and performance go, I have found the biggest performance differences come when I ....change out the stoker! But really, when the regular stoker is 'stoked' to ride, that's when we go the fastest.

If you are riding a 28 mm wide tire, I am not so sure how much aero advantage you would get with low-spoke count wheels. From the studies I have read, the narrow tire on TT bikes supplies most of the aero advantage.