Aerodynamics / Aerodynamic Cost / Aero parts

But surely the difference is seriously marginal

If @dan can do as well as the best rider in the Cervelo test, he could score 0.007m² off his C_dA by swapping from 172.5mm to 165mm. Not sure what your definition of "seriously marginal" is, but over 2% drag reduction (assuming he's already at <0.35m²) is not to be sniffed at.

Of course, he might be like Rider 1 and increase his drag by just as much, which is why nobody can give him a definitive answer without testing.