Hi,
Having a way to edit damage thresholds per bone is something that is on our wish list for sure, we haven’t got to it yet
Regarding the performance, I’d say the cluster group index solution is slightly higher on the memory side of things, but from a simulation performance (Physics thread) point of view that should be fairly similar to a single geometry collection with the same number of bones
The cost may be a bit higher on the Game thread when updating the corresponding geometry collection components but at the same time each update should be smaller than a single larger geometry collection one
I encourage you to measure the differences using Unreal Insight or other profilers to compare the real cost in your specific scenario though, as performance profiles are very specific to each project and setup 