BCM4718A1 is only supported by b43, but b43 doesn't have any kind of support for HT modes, which would be required for achieving throughputs above 54 MBit/s (and no, there is no hope for this to change in the future). If you need more performance, you'll have to look for a better supported router (there are quite a few nice devices on the used market for rather little money).
2 Likes