I agree with Safire, which (I think) may be in conflict with your criticism of the billboard-charts recap.
As you quote, Safire wrote that "you can use further to mean either degree or metaphoric distance". That's precisely what's being done in this quote:
"Sinatra topped the charts most often; Dean Martin and Sammy Davis, Jr.Perry Como further behind, a distant fourth with only six top-ten hits."
The concept of distance is a metaphor for chart ranking. We see it repeated in your second example:
"This project is just in the planning stages, the implementation phase is much further down the pike."
Here again distance is being used as a metaphor, in this case for time. Because it would be possible to use a different metaphor in the same sentence, it makes sense (to me) to follow what Safire recommends. Adhere to 'farther' only when comparing physical distances. Use 'further' for certain when describing degree. And (my own personal rule, allowed by the esteemed Safire): also use 'further' when comparing metaphorical distances.