I just had a go at this myself, using the combination of an SRM and the Lemond Wattbox. I have previously established that the two do not drift relative to each other over a long session. I went through the entire cassette, riding for 5 minutes at 230W in each sprocket, and then as a final check I did another 5 minutes in sprocket 5. I subtracted the delta for sprocket 1 (smallest) from all the values, as sprocket 1 had the highest drag, so the chart shows how much less drag each sprocket had relative to the smallest sprocket:
As you can see, I didn’t find anything close to 10W difference between any sprockets, the biggest difference is 1.4W between sprocket 1 and sprocket 5.
The repeat test of sprocket 5 gave a result 0.2W different to the first test (giving it 1.6W reduction). I don’t know why sprocket 7 looks a bit out of place, but on the whole, my testing suggests any differences are <1W provided you avoid the smallest sprocket and the two largest sprockets.