You could use RISC-V in those POWER8 applications with a suitable implementation. I don't think it would be POWER and probably be only a fraction of it. Just so much careful optimization and design in POWER at ISA and implementation level for number crunching. Yet, an Octeon III-like implementation of RISC-V esp w/ accelerators for SIMD/MIMD or onboard GPU could kill a lot of Intel and POWER processors in performance while getting us close to top contenders.