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https://www.cyclingweekly.com/group-tests/best-skinsuits-for-cycling-360975
Skinsuit grouptest. Suggests up to 25w difference from best in class to a basic skinsuit.
You would imagine the sponsor skin suit would be better than a basic skinsuit, but perhaps not, so G might be having to work with a fairly significant aero decifit.
Sky have most likely aerotested it multiple times for both Froome and G so that they can tailor it as best as possible.
You'd be wrong. Double digit watts, easily