Re-using a single SSID makes it challenging in multiple ways, as, at least with PSK2 authentication, you end up giving out the only password to everything and you can't restrict the DHCP address issued without knowing the MAC address (or other client parameters) in advance. Address restriction does some to limit rogue activity, probably most of it, but is far from robust against more sophisticated actors.
Yes, "beacon pollution" can be significant, especially if you haven't disabled legacy rates and "forced" 802.11n as your minimal supported standard (at least force 802.11g).
You can see what rates your attached devices are using in the LuCI interface (or through iw dev <dev> station dump). From the rates used, you can generally determine if they are using 802.11n. ("Inactive" links may show a very slow rate in one or both directions.)
One well-written resource on this is