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It's going to be pretty marginal stuff. I'd go for 23mm rear, looks very tight at the moment.
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'closer-is-better' (that you've disproved)
No, Look disproved it for their frame. Result will vary between frames, depending on the length and cross section of the shear layer. Some seat tube cutouts have a flat back, so the shear layer is only thin at the crown of the tyre. Some have a concave section which matches the tyre profile closely, so the shear layer is thin all the way across the tyre. Some even have a deep groove which gets the frame close to the edges of the tyre for good flow transition from the frame to the wheel, but further from the tyre crown to keep the shear layer thicker in the middle. Things are also complicated by the width of the seat tube; if it's a lot wider than the tyre, you can probably open up the gap more without dropping the flow into the hole. You can guess at what might work, and you can see some of it in the wind tunnel, but really you have to field test to see which combination is faster. If you gain 1W by thickening up your shear layer with a skinny tyre, but then lose 1W in increased rolling resistance, you've just made your bike more unpleasant to ride and no faster :-)
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Kuota
Ah, Kuota Kaliber. My guess is that the leading edge of the seat tube fucks the airflow so comprehensively that anything happening behind it is moot :-)
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umop3pisdn
gbj_tester
@pdlouche
Amazing, thanks.
So except the comfort/etc issues with running a smaller tyre, I would be more efficient running a 20mm or 23mm on the Kuota, to increase the gap from 2-3mm to closer to 10mm? Currently it's 25mm with the negligible gap, running with the prescribed 'closer-is-better' (that you've disproved).