> 1420 is the default for afaik all WireGuard implementations (including
> of course existing OpenBSD versions), I think diverging from this is
> likely to cause more problems than it'd solve

Yeah, that's definitely a good reason not to change. Apparently there
are WireGuard implementations that don't perform this padding. It's
not difficult to manually set the MTU to a more appropriate value
when you control all the peers, so I'll be sure to start doing that
even for my Linux devices.