I've installed Windows Phone 8.1 Dev Preview on my Nokia Lumia 925 in order to get access to the new features .
I was surprised to see how good is IE11 since IE10 sucked big time. One of the most important update is the support for WebGL. Internet Explorer 11's performance can now be compared to Safari on iOS7 .
Also Touch.Acceleration now works
Nokia Lumia 925 (WindowsPhone 8.1, Internet Explorer Mobile 11)
Battery Level: Does not work, shows 100
Language, Platform, DevicePixelRatio, BrowserName: Works
AccelerationX, AccelerationY, AccelerationZ : Works
AccelerationXWithG, AccelerationYWithG, AccelerationZWithG: Works
Alpha, Beta, Gamma: Works
Here are Scirra's benchmarks:
Render Performance 5240~5260 objects at 30fps (WebGL)
Particles Demo 820~860 particles at 20fps (WebGL)
As a bug (don't know if is C2 or IE11's fault), I've tried my old game Bounce Line and the text boxes (not just the text) were rendered black. Also there was no music/sounds. Except that, the performance was great.