Presumably Android will also chose a base minimum ISA spec for riscv software at some point. It would be tidy if they match the regular linux ecosystem
It has not been decided. There was a talk about the details just 2 weeks ago on the RISC-V International Youtube channel. Very interesting and recommended.
There's a lot in there which RV64GC (ratified in 2019, I understand unchanged since 2017) lacks.
Some of the new extensions e.g. vector and bit manipulation do have a significant effect in performance.