The data sheet for this device indicates a IVR 0V to 28.5V for Power supply = 30V
is IVR the max voltage for proper signal ampification?
i.e. if Vin = 28.5V, for gain = 1 then Vout = 28.5V?
The IVR you have mentioned is aligned to the data sheet.
This already limits the voltage range that can be processed by the part.
For the Vout, ideally with the gain of 1, the output will be equal to your input.
Your example is then true.
For other applications especially when the output is going rail, the output can be also affected by the VOH/VOL.
If the application is much sensitive, we have also to consider the Offsets on the voltage and current, as well as the noise.
These parameters may affect our expected Vout.
Furthermore, it will be helpful if you can share to us the actual circuit that you will use as well as the nature if the input.