I know this sounds like a weird question but.. WTF. The "cast optimized youtube experience" is just plain horrible. I would much prefer to just cast the tab as normal.
Sometime I seem to be able to do it by accident, but most of the time everything switches over to the "optimized version". I really like my Chromecast for basic screen mirroring. I see it as an awesome wireless HDMI cable where everything even syncs up perfectly. Being able to cast a specific tab works great, even in Full screen.
But when I try to use the "optimized" Youtube version it lags, sometimes "loses control" and the volume adjustment adjusts the actual volume on the chromecast, so if i stream a normal tab afterwards and forget to reset it to max, everything is too low without the ability to change anything.. It will also refuse to add stuff to my "TV queue" and... Well, the list keeps on going, and I really find the experience horrible. I tried to make a big iframe of Youtube, but they disallow that.
I don't know if anybody knows a hack to make me able to just cast Youtube as a normal tab without the "Chrome readiness" - This is a very big deal to me.