I'll second the recommendation to look at the XTR's. Lots of discussion about them in the Tires and Rims forum. Unfortunately, the XTRs are not any lighter that the 589s according to the ITP website. I can confirm the weights listed for the 26 inch XTRs are correct. The XTRs should ride smoother and the radial design should offer less rolling resistance, maybe one reason most people don't seem to notice that much power loss. Still love the look of the 589s. Also, the XTRs tread is much more impressive looking than the pictures of them seem.
Good luck,
Steve