Just saw this video on Cycling Weekly's website from a few days ago where they tested 3 different sets of wheels at 200W and 300W on Herne Hill velodrome.

How much faster are aero wheels? (video) - Cycling Weekly

Certainly some interesting numbers at the end of the test. I've added the percentage faster over the Mavic wheels for comparison.

1) Mavic R-Sys SLR
At 200W (200W actual, 201W normalized) – Distance travelled: 5120m at 30.7 kph
At 300W (299W actual, 299W normalized) – Distance travelled: 6070m at 36.4 kph

2) Enve Smart 4.5
At 200W (202W actual, 203W normalized) – Distance travelled: 5460m at 32.7 kph (6.5% faster than Mavic)
At 300W (301W actual, 302 normalized) – Distance travelled: 6442m at 38.6 kph (6.0% faster than Mavic)

3) Hed GT3 Trispoke and Lightweight Autobahn (rear disk)
At 200W (200W actual, 202W normalized) – Distance travelled: 5520m at 33.1 kph (7.8% faster than Mavic)
At 300W (300W actual, 300 normalized) – Distance travelled: 6600m at 39.6 kph (8.8% faster than Mavic)

Seriously, 6.0 to 8.8% faster is nothing to sneeze at.