I have a set of Mavic Classic Elite hubs...The rear hub spins great until I clamp down the QR, at which point it slows noticeably. Seems that the axle/hub assembly is heald together with an allen bolt on one side and a 15mm or so nut on the other. Those are already not tightened very much, but is it OK to loosen even more until just beyond hand tight?

This sealed bearing thing is all very different from the old days of cone wrenches and serviceable bearings.

Thanks

You might have the hub adjusted a hair too tight. A properly adjusted hub should have about a hair width of travel.