300Mbps is the "interface rate" ! See http://mcsindex.com/
, yellow field, 40 MHz bandwidth (use only if you are alone there!), dual stream, MCS15, SGI=short guard interval of 0.4µs)
Practical throughput could be 200Mbps for UDP, 150 Mbps for TCP. Overhead depends on the buffer size used.
Available airtime will not be 100% if other 2.4GHz devices are around. And also the quality (CCQ) will be reduced by noise and interference, what will make the devices lower the MCS rate used.