It makes sense that the universe would be the most efficient system for "calculating" itself as nature tends not to be wasteful. I would only mention the existence of analog computing. It doesn't change the conclusion of your analysis-- analog computing can provide, at best, several orders of magnitude of increased computational performance, not enough to grow exponentially with the number of particles being simulated, but enough to suggest that digital-quantum hybrid computing will (or maybe already is) occupying a niche analogous to digital-analog hybrids in the 1950's and 60's. Perhaps digital-quantum hybrid computers could more easily solve some of the multi-particle quantum interactions hinted at by the DNA and photosynthesis processes you've mentioned.
It makes sense that the universe would be the most efficient system for "calculating" itself as nature tends not to be wasteful. I would only mention the existence of analog computing. It doesn't change the conclusion of your analysis-- analog computing can provide, at best, several orders of magnitude of increased computational performance, not enough to grow exponentially with the number of particles being simulated, but enough to suggest that digital-quantum hybrid computing will (or maybe already is) occupying a niche analogous to digital-analog hybrids in the 1950's and 60's. Perhaps digital-quantum hybrid computers could more easily solve some of the multi-particle quantum interactions hinted at by the DNA and photosynthesis processes you've mentioned.
https://en.wikipedia.org/wiki/Analog_computer