There are a couple of good stores at Citygate. There is the Sunglass Hut Outlet store (Down near the Novotel) but I can't recall if they sell Serengetti's. There are two eyeglass stores, one on the third floor across from Esprit and then a place in the Fu Tung Mall on the other side of the MTR. I bought two pairs of Oakley's in Fu Tung and saved at least 200 USD over buying them in the US!
Worth a look at least!