How similar are saddles made by the same company? I have a Specialized Phenom on my mtb that is very comfy and a Specialized saddle (standard one from an Allez) that I use on my commuter bike which is also comfy.

The Selle Italia saddle that came on my Cube road bike is not and I'm looking to replace it. Can I assume that all Specialized saddles are modelled around the same shape and will fit the same (for a given size/sit-bone width) and that if I have one the right shape, others will also be or is it a case of taking every model in its own right? Will be a while before I can get time to go to the LBS to try seats.

(I know it's not really what this site is for but does anyone have experience with Specialized Toupe saddles? Worth trying?)
The only thing that you might be able to bank on is the width of the saddle.  I know specialized in particular has different widths for the same model of saddle based on where you sit bones are.

Other than that, you will really need to try each saddle to know how it's going to feel.  I have the Toupe on my road bike and it's fine for a road bike but I wouldn't want it on my MTB.  It doesn't have as much padding as the specialized MTB saddles.  

Short Answer: You have to take each model into consideration individually.