Neat! Had to do a quick search for what this does. We're going from 48-bit addressing to 57-bit addressing (256TB of addressable memory to 128PB of addressable memory).
Huh, this will affect all Nan-boxing vm's, like v8, luajit, etc. Didn't know that they even had this 5level config already. 9 bits less for additional types now. But in reality you would be fine even with the last 3 bits, as in most not-nan boxing fast vm's.