There is a limit to MGU-K output, you cannot deliver more than 160hp/120kw from MGU-K to the wheels. That said, there's still great benefit of having a MGU-H that can deliver more than 120kw. You can use the excess energy to charge the battery, or run other systems.
I don't think there's a magic bullet in any component in the car. Maybe teams are exploiting this, they're also exploiting a bunch of things you never even thought of. What if in addition to the standard armature and windings, there's thermoelectric generators, magnetic arrays, and all sorts of other neat tricks designed to boost MGU-H output as well as efficiency. Then there's the working voltage of the ERS, at present most teams aren't even close to the 1 MV limit, not even half way there. The regulations assume a 5% loss, any efficiency above 95% is extra power.
From the regulations:
Electrical DC measurements will be used to verify that the energy and power requirements are being respected.
A fixed efficiency correction of 0.95 will be used to monitor the maximum MGU-K power.