It's funny how ideas come back in some ~30 year cycles. The energy efficiency of optical computing has been discussed quite extensively, e.g. David Miller at Stanford wrote a science paper laying out the fundamental problems with optical nonlinearities for computing, i.e. phonon are bosons and therefore don't like to interact, or optical nonlinearities are very inefficient compared to electronic ones. So in that sense FWM is not efficient.

Now for some specific operations optical signal processing might still make sense, for example if you can take advantage of the bandwidth and inherent parallism.