I thought I would try Asus 3.0.0.4.376_1071 from Merlin 374.43 on my RT-N66U and the wireless range is awful compared to Merlin's build. I did a full reset and manual set up.
One room away from the router now sees my phone's 5 GHz Wi-Fi signal strength at one bar, whereas on Merlin it was three, and 2.4 GHz is now at one/two bars, whereas it was full bars before.
There are rooms in the house where I struggle to pick up the 5 GHz signal and the 2.4 GHz is weak with Merlin's build, I now reckon I won't be able to pick up either in these locations.
Throughput doesn't seem as good either. 152 Mb broadband and YouTube kept buffering a 7:34 720p video.
Since Asus moved away from the SDK5 driver, the wireless performance seems to have gotten worse with each release, turning a great router into a not as useful router. Despite all the security updates since, I am seriously considering rolling back to the last SDK5 firmware.