That is hard to say at this point. From my experience with my Jeep, the majority of wind noise is caused by the aerodynamics of the car. Wind noise around the mirros, windshield, etc. Since the Solstice isn't a brick on wheels, I am sure it will be better than my Jeep.
Since the top won't insulate the car from outside noise as well as a hardtop car would, you will hear some noise. Some wind whistle, the cars around you will sound louder, road noise from the tires, engine/exaust sounds, etc. However, I find none of it really obtrusive in my Jeep (which has a triple ply canvas top). The worst noise is wind on the highway, on a windy day, and the Sol should not be nearly as bad in those conditions due to its more aerodynamic shape.
