I have both the Alpha 77 SLT and the Alpha 900, the SLT is APS-C and the Alpha 900 is FF. Both are 24 MP. My feeling so far is that the SLT is a pretty fair match for the Alpha 900, but much more demanding on lenses. I have not done any "scientific testing", though.
The SLT has some advantage in better DoF at equal aperture and live view AF is a real benefit. Right now I use the SLT for tele work, for normal work, either does fine. The Alpha 900 has better viewfinder but lacks live view. The Alpha has a virtual horizon I miss on the Alpha 900.
As is now, I would not buy an Alpha 99 with 24 MP, as it would offer little benefit over the combo I have. An Alpha 99 with 36 MP would be much more interesting. I would also say that user interface is important for me. So would the Alpha 99 arrive with 24MP and a really excellent UI, I may buy it anyway.