Last Post: July 12, 2016:
Please help me to understand ac datarates
Here at least is a useful tool from Aerohive:
Notice that there are only 10 MCS index numbers. The actual rates of which depend on channel width and SGI. Add in multiple streams for even higher rates.
In contrast there were 77 indices under 802.11n. Part of the reduction is due to the removal of the unequal modulation rates - which were never implemented.
Also, on an /ac AP you cannot selectively choose which rates you get. The PHY/MAC parameter field is not set up for that.