ooops!!! I was thinking of the older Shimano wheels; the 540s

MAN, you wanna stiff rear wheel? Just get any old 30mm deep, 32 hole rim, such as an old Mavic CXP30, or a Velocity Deep V. They'd be heavier than your Shimanos, but I also bet the Mavic disc that Theo Bos cranks up to over 70kmh is heavier than the Shimanos! Sprinting speed is about stiffness, then aero, not weight. A couple of years ago, Robbie McEwen was using some 28-spoke things that were tied and soldered!!

I also had some 32-hole CXP33s that I thought were stiff enough.

John Kennedy is a big fan of the Campag wheel-sets.