Cat6 or cat6a?

I am running 300' of cat6 from a cm700 to an AC750 R6020 setup as a wireless access point.  I am getting about 250mbps at the modem, but it drops to 65mbps at the end of 300' of cat6.  I read cat6 could handle it, but not so much for me.  Thinking of buying a 300' section of cat6a, as I have heard it too is the bomb, but hate to waste even more money.  Any input would be greatly appreciated.  Much thanks.