Looking at this from a simplistic point of view, I would think that WBO2
would be better than IAT. Under boost the HFS-4 will already have active
failsafes to guard against loss of water flow, which I imagine is the most
likely cause of the IAT spiking.
However, if there was a loss of fuel pressure for any reason, the AFR could
spike into dangerous territory very quickly. Unless the engine management
or engine instrumentation was equiped with a warning alarm to force an
emergency shutdown under a sudden lean condition, engine damage could
easily occur before the driver realised the danger.
Than would be my reasoning. But I have no experience with water injection
or the HFS-4, so I stand open to correction
Cheers... jondee86