It's possible that there's a new chip based on N6 considering it's supposedly going to replace almost all N7 designs.
However this doesn't explain the smaller heatsink because N6 should have the same power efficiency as N7/N7P.
It's more likely that the original heatsink and PSU were overengineered, and Sony is now more confident in using a design that matches the SoC's TDP.
Theoretically, you should get better power efficiency doing the exact same floorplan N7->N6 as you'd end up with somewhat smaller transistors, thus more dark silicon. Of course, only someone with PDK access would know what the exact parameters are.