I believe that there are major incompatibilities with 360s and 1st-gen 802.11ac routers. The short version is, don't use either Asus RT-AC66u or Netgear R6300 if you want to stream video from a Windows PC wirelessly to a 360s, ESPECIALLY MCE. Yes, I know the 360s integrated wireless is never going to take advantage of ac, it is 2.4GHz N, but the point is I couldn't even get 10Mbps out of it with these two routers. The 2.4GHz (N) support in these routers is poor IMO.
I posted a while back(1 yr. or more and cannot find the thread) that I was having major issues with my (4) 360s consoles that I occasionally game on but mostly use as Media Center Extenders. This started when I upgraded from a Netgear WNDR3700v2 (dual band N router) that worked fine for streaming(I wanted more speed for sharing an HD TV tuner to a wireless PC in my kitchen though) with either Netgear firmware or DD-WRT.
I bought both the R6300 and the RT-AC66u as they were the hottest thing (just)out and supported new 802.11ac. The consoles dropped basic connections all the time and streaming MCE was basically a black screen with some glitchy audio before the whole thing just stopped. Tried all the latest firmware, and even DD-WRT on the RT-AC66u, changed every setting I could find(even those I know would make no difference) just to try to make a difference, ANY difference. Not once did these devices act right together. Back to the WNDR3700v2 and all was back to "normal," with normal but infrequent wireless headaches.
Two weeks ago I bought the Netgear R7000 and I can tell you it has been FLAWLESS. I have had (4) 360s consoles all streaming different live HD channels(HBO, Cinemax, Showtime, and Starz, for example) at the same time and not even a hint of lag or drop.
Again, I know that the Xbox doesn't support ac, but what I am telling you is that the implementation of 802.11n/2.4GHz on this router is fantastic and worth the $$ even if you don't have/require 802.11ac.
The guys in this forum gave me some good advice on things to check when I originally threw my hands up over this issue so I figured I would contribute in turn since I /kinda/ figured out my problem. Anyone have questions, feel free.. My setup is:
(3) 360s mfg date ~09/2011, (1) 360s mfg date ~2010. Netgear R7000 with latest NG firmware, Ceton InfiniTV 6 (Ethernet version), Windows 8 Pro with Media Center. All working /really/ well for the first time in a long time I think.