One way I could see an 8 core Zen implementation work out TDP wise is asymmetric clocking of the modules (Zen comes in modules of 4 cores, 512 kB L2 per core, 8 MB L3 per module).
An "app module" (for running Scorpio exclusives) could be clocked at 2.4 GHz or higher, while the other one, an "OS module" could churn along at 1.8 GHz with two cores always idling or uninitialized. A setup like this would facilitate both ease of development and decent performance gains by:
-freeing up memory interface resources (only one module heavily accesses the RAM, no communicating between the modules via RAM, bigger cache);
-it's easier to schedule jobs/threads across 4 cores with shared LLC than 6 without;
-higher clock, wider execution units, faster cache, etc.
Alternatively, booting up legacy software (or a cross generational game) would clock both modules evenly at 1.8 GHz and enable the two idle cores (on the OS module) which would allow XBone titles to run natively or with minimum modifications to the executable.