gddr5 not overclocked in most modern graphics cards is as fast as some of the fastest overclocked ddr3 but i am unsure about the CAS on the GDDR5 but with that said they work in different ways ddr3 and GDDR3 worked the same but GDDR3 works on speed while GDDR5 works the same on a lower speed because of the higher bandwidth so a better question will be why don't we improve the bandwidth of the RAM standards?