If you let the chip actual boil enough water to run a turbine you're going to have a hard time keeping the magic smoke inside. Much better to run at reasonable temps and try to recover energy from the waste heat.
That's basically the principle of binary cycle[1] generators. However for data center waste heat recovery, I'd think you'd want to use a more stable fluid for cooling, and then pump it to a separate closed-loop binary-cycle generator. No reason to make your datacenter cooling system also deal with high pressure fluids, and moving high pressure working fluid from 1000s of chips to a turbine of sufficient size, etc.
