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PHY (11n vs 11ac) and Channel Width (20 vs 40 vs 80 vs 160) is just two components to determine the max data rate for a 802.11 wireless client.
Other components include 1. # of Spatial Stream (1-4 for 11ac) 2. Guard Interval (Long vs Short)
Typically when you double the channel width 20 -> 40 you will double the maximum data rate + some small factor. For example 3SS VHT/11ac 20 MHz Max Data rate is 288.9 Mbps as compared to a 40 MHz which is 600 Mbps
This chart might be helpful for you to see the Maximum Data Rate for each 11n/HT and 11ac/VHT http://mcsindex.com
Note the Max Data Rate is not the Maximum Throughput due to nature of 802.11 (half-duplex shared medium) and 802.11 overhead frames (control and management)
Also you can see different throughput measurements depending on type of protocol (TCP vs UDP)
Many many factors come into calculating max speed and it all depends on the client just as much as the Access Point. For example a VHT80-SGI 3x3:3 11ac Access Point has max data rate of 1300 Mbps (3SS-MCS9) but if a client is a VHT20-SGI 1x1:1 the max data rate that the client can achieve is only 86.7 Mbps (1SS-MCS8).