Checking to compare A99 vs A850, I noticed the real iso measured on the A99 are vastly inferior to the nominal one and inferior to those of the A850.
If you take this in the right account even considering the iso 50 setting not available on the A850/900 the noise performance of the A850 equals the one of the A99 up to the 6400iso barrier (with reference to raw files only).
The only real advantage is the greater dynamic range of the A99 since the gain in colour depth is less than marginal. The above just refers to sensors comparisons done by DxO obviously the A99 adds many feat. vs the previous generations of Sony DSLR FF. Anyway since I'm interested on the photo side only, I don't see this new camera as a real step up from my trusty A850 which raw files are a pleasure to work with DxO.
I will buy tomorrow an updated version of the A850 with an integrated grip plus live view for photo only, multi frame noise reduction, in camera HDR, peak focus and the panoramic capture. Such features are a joy to use on my second body the A65