Harvesting from gathered energy could became weaker (less inertia, less stored rotational energy)?
I don't agree with this. It all depends on how you harvest the energy. You are right that with less mass you can harvest less kinetic energy. But this is not about harvesting kinetic energy. You want to harvest waste energy.
The way i see this working is that you use the MGU-H as a harvester when maximum boost is reached. You use the MGU-H as a generator to control the boost. This is why some of us including me thinks that wastegates are redundant.
So at 10.000 rpm when max fuel is allowed this rpm should be when max boost is as well. From 10.000-15.000 boost should drop to compensate for the increase in air flow the engine creates itself when increasing revs. So you brake the turbine with the MGU-H and harvest the excess energy. The rotating mass only has something to say in the time between to fixed numbers. ect. when you brake the turbine from 125.000 rpms to 100.000 rpms. The time it takes for the turbine to get from speed A to B is the time where you would be able to harvest more energy with higher mass. But i dont think the MGU-H will be used to brake the turbine like that. It will be used to maintain a surtan rpm or slightly reduce rpm after 10.000 rpm engine speed.