I think the broad point is that in terms of sensitivity and signal to noise, a physically smaller pixel will always be disadvantaged relative to a larger one, at least at the scales we are talking about. The question is at what point does the performance of the smaller pixel become so good that we no longer care about the "better" performance of the larger sensor. Arguably, the best 4/3 sensors are approaching this limit now at around ISO 3200/16 MP.