I only recently realised I could use this feature, and turning it on reduced the CPU time for the OvrAvatar update loop from 1.5 ms to around 0.8 ms - saving around 0.7 ms per avatar on an S6. If you don't know, it translates C# to native binaries. There is a lot of really slow MS native inter-op code in the Avatar SDK that seems to have benefited a lot from that.
Hopefully this helps someone else. Make sure you use a late-2017 or newer Unity as there were lots of bugs floating around before then I think.
PS: Another tip for lowering processing on lower end devices is that if the user does not have a controller connected, and is alone in the scene, you can disable OvrAvatar completely and save a bit of time, since the avatar is likely not doing anything in the scene at that point anyway (unless you have other cameras for some reason).