Dude, I have the EXACT same issue. My iPad 3 and nexus 7 load up HD just fine. The Air sticks at low res. If I reboot the iPad, it works sometimes. But more often than not it sticks with low resolution. I have a 100mbit line as well with an Asus AC router....bandwidth is not the issue. If anyone can figure it out let me know. It's just weird that the iPad 3 has no issues with this and it has the same version of iOS.
Run a speed test on the iPad. I have very fast internet but wasn't getting the HD stream. I am now after the network settings reset.