I don't have specific weights, but yes, I am 100% sure that a painted Polyspan finish will still be much lighter than Solartex.

The tail surfaces of my Gotha G.IV are built up from 1/8" X 1/4" balsa sticks, and the polyspan didn't warp it. It's still a good idea to try and shrink the covering as equally as possible, and also to try and dope both sides at the same time. The dope will taughten the polyspan a bit further, so you want to avoid unequal shrinking on delicate structures. On normal built up wings and fuselages, there's no problem with warping.