I went from 2.1 knobbies to 2.0 slicks to 1.75 inverse tread (like the Marathons) on my MTB before deciding to repurpose it as a tourer. Going from knobbies to slicks made FAR more of a difference than dropping that next quarter inch. Can't speak to 1.5, but the difference between the 2 and 1.75 was negligible, to the extent it played no part in my latest tire decision.