Hi,
I have used ASITIC for several applications and find it gives accurate results for simple on-chip inductors, accurate in terms of both inductance and Q. More complex structures, such as tapped inductors, are not so accurate but still give reasonable results. An important aspect to the accuracy of the simulation is the data provided for the various layers. These values should be in the process documentation.
To make a single ended to differential output balun requires one input to be grounded. Also, the inductor will consume quite a large area of real estate. You might want to consider other ways to connect the LNA to Mixer. One way is just connect one mixer input to ac ground. There will be a linearity penalty but perhaps that is OK for your system