I run all my C7's as APs and use 80MHz wide 5GHz channels only. With stock 720MHz clock, you run out of sirq before you run out of bandwidth. 1GHz is a sweet spot where both WiFi bandwith and CPU grunt converge...it runs flat out on both and nets 500Mbit of WiFi throughput (no NAT, just WLAN).
But even if you run it as proper router (I use x86 for that), performance scales in linear fashion. OpenVPN is faster, routing goes quicker etc. 1GHz is sweet spot. No heat or clock issues. More is possible but internal clock usually skews beyond 1GHz.
I use BREED bootloader for OC: Archer C7 5GHz performance, sirq 99%