Easy way to check - play "Example Short 23.976" if you get 4300 or 5800 kbps you are getting 'Super HD'
The bit rate is displayed on screen with this 'interesting' title.
I may try later if I get a chance.
Edit - just checked - yes it ramped up to 5800 kbps in about 15 seconds.
So I am getting Netflix Super HD on my Chromecast through Comcast.
Oh, I see you tried it since while I was typing up my post.
Yes, I tried too and since I only have a 6 Mbps max internet speed, I get 4300 kbps after it ramps up.
This is the same bitrate I get on my Samsung Smart TV that has verified Super HD with the little logo in the descriptions.