Velocity Thracians. Love them on my Baron. They're not stupid-light but they're lighter than stock, they're strong, and they roll great. The low spoke count really makes a difference at speeds over 15 mph.
I agree, the RANS seat is pretty heavy - I think mine is 6 or 7 pounds counting the heavy upholstery foam pad in the base. I don't know what else to recommend though; if I did, I'd probably already have replaced mine.