WiFi 802.11n goes up to 600Mbits/s.
So find a good, supported USB 802.11n dongle with an external antenna and you'll have the hardware that can achieve your 300Mbits/s (and more). You'll want to use it in a powered hub as it's going to need the full USB spec 500mA.
You'll then find that the real networking bottleneck on the RPi is the USB interface, it's, simply, not fast enough and there's no way to improve it. Because the on-board eth0 is driven off the same USB interface chip it's never going to go very fast either.
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