I thought it was 3dB = half.
Extra antennae besides the internal 2.4 GHz and the two external 5 GHz antennae? Also, if you want gain coverage with 5 GHz, like @SAn says, those are used by default for mesh. You would have to replace LibreRouterOS / LibreMesh with OpenWrt in order to use the 5 GHz radios as access points. If you did so, I wonder whether 802.11r fast transitions would make sense to manage connections. I don’t know much about it.