Which DRAM type is the main component of HBM? Could it benefit from the development of GDDR5X?

Whenever I read about the structure of HBM, it's never specified what kind of memory makes up the bulk of the stack, it's always just called 'DRAM'.

My best guess would be a mofidied version of GDDR5 that's suited to the TSV structures and HBM protocols. If that's the case, would it actually make sense to use modified GDDR5X for the stacks as soon as it's ready or would something prevent additional bandwidth gains?

On a side note, how much does memory bandwidth limit graphics cards these days and do you think could vanish as a bottleneck in the near future?