After using the A99mk2 for a several days, her high ISO performance is most impressive for me. In A7RII, personally, I will not use ISO higher than 6400, after that you will see drastic increase in noise and lose a lot of details especially in real dark situation.
Apparently, the senor of A99mk2 is very similar to A7RII (both are 42mp backlit sensor). However the high ISO performance in A99mk2 is significantly better. I purposefully take several test shots outdoor, a very dark situation and the lighting is very challenging which can push the dynamic range of the camera to her limit.
Those pictures are taken in ISO6400, 12800 and 25600 and converted by Sony IDC (totally untouched, the dynamic range adjustment has been disabled). In 6400 and 12800, i think that is absolute useable. in 25600, the background noise become obvious however you can fix it easily in LR (however LR still can’t open the A99mk2 RAW !!)
Of course this is not a scientific test, with no professional equipment to measure and handheld etc….but i think it is strong enough to convince me, as a user of A7RII, the A99mk2 is doing much better.
I try to test the dynamic range of A99mk2, and see if it can retain every detail of an underexposed photo (from -3ev to -5ev) . However I can only boost ev up to 2ev in Sony IDC, so….maybe i can do it later when LR finally supports A99mk2 raw.
I do think the true performance of A99mk2 is yet unveiled. Later when the LR can finally convert A99mk2 Raw, i believe we will be surprised even more.