DxOmark tested the A99II sensor performance. While it scores a very high result it cannot match the Sony A7rII performance. With the SLT mirror you gain speed and autofocus performance but the trade-off is the small loss in light gathering. The verdict from DxO:
Although better color depth and dynamic range image quality at base ISO is available on the Sony A7R II, photographers after a larger DSLR style body might prefer the A99 II’s design and handling over the mirrorless A7R II, and its overall image quality isn’t far behind. What’s more, the A99 II’s fast 12fps burst shooting with autofocus tracking puts it on par with high-end professional sports cameras, and its BSI sensor technology ensures good low-light performance despite its massive 42Mp resolution. The SLT fixed translucent mirror and resulting EVF might be a drawback for some photographers used to an optical viewfinder, but this brings other advantages for some subjects and in some situations.
You get an awful lot of camera for your money, making the Sony SLT A99 II a serious contender for high-resolution shooting.