I'm confused by the comment that Nios II works at 600MHz. I do not believe that is possible. Beyond that, generally the SOPC system(not just Nios) will limit the Fmax. Hooking up peripherals, etc. makes a large mux for data/address, which is slow.
475MHz is just over 2ns clock period. Once you start looking at paths in TimeQuest with report timing, you'll find that are generally longer than that. Stratix IV is the fastest FPGA fabric at 40nm(IMO), but what you're trying to do just doesn't seem to be close to possible.