Looks like a solid recipe. Based on Marshall's description, specifically the "toffee" part, I would suggest subbing out the American Crystal 80 for a combo of English Amber Malt and English Medium Crystal. I think the English malts give more toffee flavor. Fawcett Amber and Simpsons Medium (60L-ish) would be great together.
I forget the IBU specs of a Robust Porter, but 53 IBU seems pretty high. I like the idea of NB as a FWH, especially when you're adding NB as a late kettle addition-- you get the full spectrum of flavor from the NB that way. Perhaps keep the NB hops as-is and adjust the Magnum hops in the kettle (not FWH) to your desired IBU. Magnum will not give you much of what you desire in a FWH. It's pretty neutral (in normal dosages) and you're just wasting IBU's if Magnum is a FWH. I would say use them in the kettle only to adjust for IBU.
My $0.02.