I did a bunch of research before buying my wheels, Mavic Ksyrium Elite. I knew they were pitiful aerodynamically, but also that it didn't start to matter very much until beyond 25mph. At the time I was happy to average 13mph and excluding descents, I was really working to do 20.

Fast forward a year and I'm doing interval workouts with a buddy who has a power meter. We're always going well over 20 unless we go straight into a strong headwind. We average 30 mph with a big tailwind.

I'm not trying to say I'm fast. I'm not nearly fast. But I'm now going fast enough for some regret to creep into my head that I should've at least gotten 30mm aero clinchers instead of box-section wheels.