Last night I tried a 30/32 Roubaix Pro Tire in the back for comfort. I did not like this combination. Robbed the fun out of it for me, felt so sluggish. So I exchanged it for a 25mm Conti 4000S GP. Restored the " spirit" of the bike back. Different tire combination/sizes work for different bikes.
Still have the 25/28 in the front and works terrific there, for the traction.