Bob, IIRC, I have. It didn't work too well. The foam is really too soft.
With flex track, I strongly prefer WS foam.
For hand laying, cork.
My current layout is a mix of straight on pink foam and cork on pink foam. I definitely feel that cork on foam is the better of the two (all is hand laid). It is easier to spike it and it does a better job of distributing the weight (remember, real roadbeds are supposed to spread out the weight...notice how real track floats up & down when a train passes over it?)
Cork or foam loses some of its sound deadening when coated in glue and glue to plywood. I recommend adding a foam base in between the roadbed and plywood. It is far quieter and prevents the vibrations from being transferred through the ballast to lumber. This way, the foam is a sound absorber while the foam shields the plywood from the vibrations. I'll also go very easy on the glue when it is time to ballast. Similarly, I've heard of John Eichman using a double layer of roadbed on his Proto:48 layout to make it silent.
As far as the width (and the height), just check out the real thing. It is too high for yards, but that's why their are sheets available. I recall it being the same width as Midwest Products cork.