I got a MyBook NAS and can get about 80Mbps from a wired Gb PC via a superhub on a good day with the wind behind.
I find using the 2.4ghz band that 300mbps is actually slower than 145mbps.
I find using the 5ghz band that 300mbps actually works as it should and provides the fastest LAN transfers.
Try optimising your channel selection on 2.4ghz if you can't use 5ghz and make sure all your devices are on the same "page". Personally I have all mine set to auto negotiate.
Even better still, get a router that works properly and set the superhub to modem mode.