I've field tested quite a few shoes / covers. Most marketed as aero. (although not Crono's) bont zero's, spec sub6 (without the sleeve) DMT P1's, some old Spec S works shoes I removed the boa's from and made into lace ups.
For the most part, they were all very close, within what I'd consider my margin of error. Covering the shoes (velotoze short) seemed to be a positive for all of them, but again, narrow margins.
I don't know about 10+ watts on a "normal shaped" shoe, even going from an ugly one to a smooth one. Maybe if you pedal flat footed or toe down and that shoe specifically mates well to your style.
I had expected GP's custom Simmons shoes to have been a big improvement (although the more I look at that design the more I'm not sure about that) but from his data it was almost nil.
I think, for the most part, the foot is not a good shape for being slippery. The farther away from that you go, likely the better. (why the Crono / old Project 96 shoes are likely better generally)
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