I have 17" ZIEX 512 and as I have harped on before, these tires were rated #1 by consumer reports issued last November.
The GRB's have the AA traction which is the best, and this "silica combined with specially linked carbon molecules in the rubber..." that Sumitomo keeps bragging about is a reinforced filler. The presence of silica is a good thing but hard to incoporate in the rubber matrix but when incorporated successfully, you get a tire with reduce rolling resistance which in turn leads to increased fuel economy.