Ah, OK we have a thread for such measurements, so if you have the details, maybe add them here:
As I said, I just saw it in the OpenWrt wiki and thought I mention it even though I did not confirm it,, your explanation makes some sense, thanks.
Some things n networking are inherently single threaded (at least in Linux) so if multiple things like PPPoE en/de-capsulation and ethernet interrupt processing pile up on a single CPU that might limit you, and that is where irqbalance might actually help (or manually assigning some important IRQs to different CPUs) but that is not guaranteed. It tends to be a safer bet to use a router where the main CPUs are beefy enough to handle the load (and even x86 routers can be power frugal if mainly idling around). But that clearly is your decision to make.
(I run my 7520 as bridged modem under OpenWrt and am quite happy with that, and am contemplating of also using its WiFi to get better coverage, but so far did not do that yet.)