MCS (Modulation and Coding Scheme) is used in 802.11n and 802.11ac to achieve higher data rates (previous standards only allow max 54Mbps). You should not disable any of MCS data rates on a wireless controller.
Refer below blog post to understand what throughput you can achieve with different MCS rates
It is generally recommended to disable 802.11b data rates (1,2,5.5 and 11 Mbps) and low OFDM rates (6 & 9 Mbps) on both bands. Here how I have configured it in my environment.
Here is 2.4GHz band setting.
Here is 5Hz setting
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In your 802.11a network, if client association rate is 12M, and AP send control frames in 24M ( control frame send in max mondatory data rates).
Whether the client can recive the control frame?
"Access points running recent Cisco IOS versions transmit multicast frames at the highest configured basic rate and management frames at the lowest basic mandatory rates, can cause reliability problems. Access points running LWAPP or autonomous Cisco IOS should transmit multicast and management frames at the lowest configured basic rate. Such behavior is necessary to provide good coverage at the cell's edge, especially for unacknowledged multicast transmissions where multicast wireless transmissions might fail to be received. Because multicast frames are not retransmitted at the MAC layer, clients at the edge of the cell might fail to receive them successfully. If reliable reception is a goal, multicast frames should be transmitted at a low data rate. If support for high data rate multicast frames is required, it might be useful to shrink the cell size and disable all lower data rates.
Please refer the following link also :