If you are looking at a flat table layout then the approach sounds solid (I agree about drilling through it, you may want to get a sample of both materials, lay it onto a scrap piece of wood, and see how easy it is to drill through).
I did something different, I went with 1/4" plywood over a ladder frame, and then flue 4" of the blue foam on top of it. I wanted to be able to do below track level contours (water, rivers, an abandoned rail line that goes under the real tracks), and to me this was the easiest way to do it (I could have gone open grid but I have never worked with that, was a little intimidated with the cutting involved). Could be I'll be cursing at it, will let everyone know
