Call me sceptical that backside illuminated sensors are going to give a massive boost to performance.
They've already delivered a huge boost to performance with UWA lenses and lenses with a short flange distance, particularly those of a retrofocus design. BSI will only deliver a small boost to ISO performance, but are far less sensitive to angle of incidence than standard sensors.
You won't get much better high-ISO noise performance without a leap in sensor technology anyway (e.g. something that generates photoelectrons more efficiently than silicon) - read noise is already minimal, with noise dominated by photon shot noise, and microlenses are already very efficient at directing all the incoming light onto the photosites. Without a new, more sensitive material, the only way you're going to get much better quantum efficiency is by weakening the Bayer filter and sacrificing colour sensitivity.
I would not say that many of the weaknesses of mirrorless today are down to looking to create small cameras(obviously some handling issues are) but rather the reverse, mirrorless looks to sell itself on small size because the tech hasn't advanced enough to sell it purely on performance ti most users.
The tech's there. I use lag-free electronic viewfinders every day in a medical setting. Fast-focusing on-sensor AF systems are commonplace in top-level single-operator camcorders. But these things don't fit in an A7-sized body. A D810-size body, maybe.
That does also I'd say hint at a logical fallacy we often see from those pushing mirrorless or rather pushing small flange distance mirrorless mounts. That is the idea that because mirrorless currently looks to sell itself on size(honestly or not) if mirrorless tech takes over the market then everyone will desire a very small camera body. Now there is obviously a market for very small cameras(although again I think FF mirrorless can't deliver this with a larger lens system) of course BUT again the reason why mirrorless is targeting this is because it can't beat the DSLR at its own game at the moment.
I'd say it is a flow-on effect of the initial marketing push of mirrorless cameras ('SLR image quality for the size of a large point-and-shoot') that led to the current situation, rather than any deficiency in the technology. There are mirrorless systems out there - used in industry, scientific and pro cinematography settings - that are just as fast as SLRs and have imperceptible levels of viewfinder lag. But they're all SLR-sized or larger. Mirrorless was sold on its small size, so that's where they've been pushing consumer models. After all, that's an area where SLRs can never compete - from a sales perspective, why not fill the product gap first before tackling the competition head-on in their own turf?
For me, I don't care about size - I want capability. If they can deliver that in a small package, great (e.g. A7rII for landscapes and non-action). If they can't (e.g. for wildlife) then I'll take a bigger camera that can deliver. Small and light is good insofar as performance is not sacrificed. But, given the choice between a small, light camera and another camera that's larger, but more capable in the areas I need, I'll take the larger one every time.
