I've been thinking a lot about the mix of the fast lens and the sensor's ISO. It seems to me the fast lens are limited by physics about size (and hence, weight). A f2.8 500mm lens has to be large and heavy.
But, if you have a new sensor that doubles the ISO, that allows you to go with a f3.5 or 4.0 lens. Depth of field will be marginally affected, but the the size and cost of your lens will be significantly less.
What I'm saying is image quality (noise and a lot of other metrics) and ISO from the sensor is where the camera makers will/are placing their R&D resources aside from the VR capabilities.
I would would love to read an in-depth comparison of the sensors in today's bodies. If anybody knows of an article, please post the location.