Sorry to quote such an old post, but I do experience the same problem...
I have 2 routers, Linksys EA8500 and Phicomm K3. EA8500 has an IPQ8064 which is much like R7800, and K3 has BCM4709. They all have the same problem as dissent1 described in that post (reproducible by all WiFi and wired connections). Data streams going out of the router are capped at 300Mbps (<100Mbps for WiFi) but streams flowing into the router are just fine.
EA8500 is running at revision r49xx which was built several days ago without any additional patches missing from master, and K3 is running at an old build made in May. Source code for K3 is uploaded here, I have done basic porting only so it was also a "clean" build.
Maybe we can make a wild guess that something went wrong in common logic in kernel, since different platforms and different PHYs are affected...