setting 80MHz by default IMO is kind of silly because most people still dont have AC devices, and if they do have AC devices, 40MHz AC @200mbit per link will be good enough for 90% of scenario's
and theoretically it should slightly improve range
i also just wanted to note that the 600 and 289 speeds are also using 256QAM to get those speeds
600 is basically the equivalent to 450N 5GHz @40MHz 64QAM
but its 600AC @40MHz 256QAM for ac devices
20MHz without 256QAM /N would give you a max of 216.7mbps
keep in mind that
600/450
289/217
are 3x3
1x1 will respectively be 200/150 and 72/96( not 100% on that number)
and 2x2 400/300 and 144/192
so since its an AC router, if your gonna use 80MHz at all, you need to have AC, and IF you have AC, you will honestly probably not even need the 80MHz for the higher link rates and smaller channels will help minorly improve range
i do not agree with 20MHz AC mode being default however i do agree with 40MHz AC mode being default
its within the AC spec that in order to be certified they have to be able to change the size of the AC channel dynamically on a packet per packet basis, this means that if interference is detected when in 80MHz or 40MHz mode the radio will step down to 40MHz or 20MHz if required in order to avoid interference with neighboring networks
so in theory if implemented properly and in a slightly different fashion it is very realistic to have a 80MHz auto mode, however have the radio sitting at 20MHz when no clients connected, then pop upto 40MHz or 80MHz when clients that support those bandwidths connected
this could be a huge saver in the future, imagine how many wifi networks are up and interfering but not in use at the same time, even tho congestion is not as bad when neighboring networks are not in use this could imrpove congestion in dense apartments and townhomes etc when in 5 years everyone and there grandma has a 5GHz ap (or TWO!) from there ISP likely configured incorrectly..
