Solved: How to Sweep NMOS4 Device Parameters in LTspice IV?
The syntax of the step Spice directive is correct, however I misinterpreted the use of the period character. 

I have changed the directive to read ".step param w list 20u 80u 320u" and got the intended results.

The problem was that with the period the values were being read as 200n, 800n, and 320n respectively. This is smaller than the model I was using allowed for which resulted in the error.