Hard to imagine what they could be adding to CSS3 transforms - composite transforms can be done efficiently without calculating them manually anyway (by using the DOM tree as a "scene graph"). I experimented with this some time ago as well [1]
But even bigger question than the performance is, what kind of 3D user interface would be more effective in conveying a message than a clean, regular 2D website that everybody feels familiar with. Curious to see if they will bring some UI design innovations here