The new Comprehensive Plan has a new subdistrict of CMX-5 (old C-5), called "Super CMX-5", which has a FAR of 1600% (allowing 16 stories with 100% lot coverage) along the Market Street corridor in Center City.
...This is around 2/3 of the existing C(MX)-5 designation, already concentrated in Center City.
It makes sense to think of it more as a new ceiling zoning designation (CMX-6?), which allows us to start thinking about expanding C(MX)-4 and -5. These significantly denser zones allow us to upzone around subway and Regional Rail stations, especially in extant commercial centers, thereby offering (obliquely) a tool to create and leverage TOD.
In other words, "Super CMX-5" (=CMX-6 for all useful intents and purposes) allows "normal" CMX-5 to be applied at e.g. Broad and Spring Garden, Broad and Washington, Broad and Girard, Broad and Lehigh, Broad and Erie, Front and Girard, and so on. It also allows CMX-4 to be applied along Chelten Ave. and 52nd St., and around Frankford, Olney, and (maybe) Wissahickon Transportation Centers.
Instead of concentrating all density in Center City, it allows us to spread it along our mass-transit system, thereby creating more use, more ridership, and (eventually) a tool for developing and re-developing along new mass-transit corridors.
(Which brings up the issue of demand- vs. development-oriented transit, but still...)