Well, it's just like that with new drivers: they are far more stable but offers slightly less throughput the previous drivers. Well, I'm running 16.8.0.6 beta on my 7260 N but I don't notice that much throughput degradation compared to 16.1.5.2 / 16.5.3.6 drivers. Maybe it's because my internet plan is only 15 down / 1 up but LAN file transfer (60 Mbps average) with the beta drivers give similar speeds to the previous drivers at a good signal strength of -47 dbm.
Can you make sure that you're running WiFi N by enabling HT Mode with the beta drivers? Also make sure to check for the connection speed (at least 144 Mbps or greater) in the WiFi connection status..