Achieving a 450 Mbps data rate in 802.11n requires specific features:
Short Guard Interval (GI):Reduces the interval between symbols from 800 ns to 400 ns, increasing throughput.
40 MHz Channels:Doubles the channel width compared to 20 MHz, allowing more data to be transmitted.
3 Spatial Streams:Utilizes multiple antennas to transmit separate data streams simultaneously, significantly boosting data rates.
While frame aggregation and transmit beamforming enhance performance and reliability, they are not prerequisites for achieving the 450 Mbps MCS rate.
Reference: CWDP-305 Official Study and Reference Guide, Chapter on ProtocolandSpectrum Analysis

Each SSID broadcast by an AP adds additional management frame overhead, such as beacon frames. These beacon frames are transmitted periodically by the AP at the lowest mandatory data rate (e.g., 1 or 6 Mbps), which consumes airtime that would otherwise be used for client data.
According to CWDP-305:
"Each SSID increases beacon overhead. More SSIDs lead to greater airtime consumption by management traffic, which in turn decreases available throughput for actual client data traffic." Reducing the number of SSIDs is a best practice to minimize management overhead and optimize throughput in the wireless network.
- Reference: CWDP-305 Official Study and Reference Guide, Chapter on Infrastructure Design