I've had this issue as well, though, I have a XBR-65X850D and a Yamaha RX-AV, I assume it is the same as they are pretty similar models. In my case, I can't tell if the audio is behind or early and sometimes it seems fine but its only with the TV's apps. Dont notice it with the cable box, or 3 game consoles (none of them 4k unfortunately) we have connected to the receiver.

I was thinking of trying an optical because it works with light so maybe it's faster? But I hesitate to give up the convenience of ARC and confuse the rest of the household. After reading this post I am even less convinced that would doing anything anyway.

Did you ever find a solution? If not, perhaps the best thing to do is stream from another 4K device like a Blu-ray player or steam box or something.

Perhaps receivers are just not good enough at working this way yet and it would be better to just use another device like a bluray player that has apps or a streaming device?

Maybe its understandable that there might be a slight lag considering receivers are more designed to take the audio and video in at the same time and then pass it out to the TV and the speakers. Where as with the configuration with the built in TV apps, the TV is the source displaying the video and sending the audio out to the receiver and then the receiver has to process that and send it to the speakers...

Then again, I've seen people hook up things like DVD players by having the video (composite, component, HDMI) go to the TV and the audio(red and white, optical) to the receiver.... So what AM I rambling about? I guess my theory is just that the video originates in the TV, so the picture doesn't have to go anywhere because its already at its destination and can start doing its thing. The audio, on the other hand has to do a little traveling an leave the TV, go the the receiver, be processed and then go out to the speakers. Then you add ARC and HDMI control on top of it and thats a lot of traffic going through one HDMI cable... 

I wonder, if you used an antenna on your TV if you would run into the same issue...

sorry for rambling like a mad man