The only reason for offering in the 60, is the same reason Tesla temporarily offered the 40 - so they can offer an entry-level Model S around $60K (after rebates).
A P100D costs around $40K more than a 100D. A fully loaded 60D costs around $95K - and, assuming you could put the performance hardware on a 60D using a 75 battery pack, would people really spend $135K for a car that would have under 200 miles of range, with acceleration that is a small improvement over a fully loaded 100D with 335 miles of range?
Ignoring the technical details, the price points don't make much sense.