IMHO a "properly semi physical" setup for metals requires colored reflections only, plus (same colored) specular because Poser reflections don't handle direct lighting from spot/point/etc. The trick is to set the right specularity value to match the reflectivity. Metals don't have diffuse or anything alike by nature, but the reflections do introduce a mild Fresnel effect (mild - because reflection is very high already).
As a result, metals in scenes do need something to reflect, lighting included :) and Firefly needs to be set up accordingly.

On the other hand, one might like to fake the physics to speed up rendering and to simplify scene construction and lighting. After all, Firefly is a non-physics based biased renderer. Then one can throw in Diffuse, reflection maps and whatever, and one can drift away from "real metal" as far as required. 

And... one can choose to merge both worlds in order to get a shader that sort of works in all conditions. This gives about Snarlys setup. Nothing good or bad about any choice by the way, because finally we don't have to make metals, but we have to make renders which show objects that look like metal convincingly. Most people hardly can tell the difference between fake / non-physical and real / physical anyway.

I'm interested in the resulting setup.

- - - - - 

Usually I'm wrong. But to be effective and efficient, I don't need to be correct or accurate.

visit www.aRtBeeWeb.nl (works) or Missing Manuals (tutorials & reviews) - both need an update though