most differences in rolling resistance between true slicks and the semislicks like the conti travel contact, the conti tt2000 and the like seem to be pretty minimal.
Look at the conti town and country. available in 2.1, with an inverse tread, the only rubber touching during straight forward motion is a slick center strip.
anyway,
I've noticed little difference between the conti sport contact, the 'true' slick you mention, versus the town and country or the travel contacts, etc.....