The 2000MHz clock rate you are seeing in the monitoring software should be the same as the 8000Mhz rate advertised for the graphics card, just reported in a different way. GDDR5 actually transfers four times as much of data per clock cycle, so while the chips themselves are running at 2000MHz, the "effective memory clock" works out to 8000MHz.
It's similar to how desktop DDR RAM might be advertised as DDR4-3200 speed, for example, but the RAM is actually running at 1600MHz. It can transfer two pieces of data per cycle though, hence the "double data rate" in its name.