Can now load the game as ArchiveXL etc have relead updates.
Tried out the Hybrid CPU mode and my findings:
P-Core Priority:
Auto (default):
With it set to P-Core priority I can see that even in-game the E-Cores are basically parked or single digit utilisation, there's loads of hitching and frametime stuttering. The OS in the BG is basically unable to use them for BG tasks, which is the whole point of E-Cires when gaming.
When left to auto, E-Cores are used properly and all P-cores are also utilised effectively. Total CPU package utilisation also climbs back up to around 49% whereas with it on P-Core priority it drops to the 30s.
I also noticed the GPU utilisation drops to the 80s at time when P-Core priority is enabled, whereas it's 99% when on auto.
It seems this mode is not suitable for 99% of people who have no problems on the default auto. Enabling it seems to disable the effective use of E-Cores by background tasks, which are forced to use the P-Cores whilst you game, hence the stuttering people are seeing. Intel's Thread Director can't allocate cores properly when set to P-Core priority, maybe a conflict is happening as Thread Director is trying to allocate workloads to the E-Cores, but the game is stopping the use of them etc.
TL;DR - The setting is broken, leave it on Auto.