I'm not sure where he got the math that available energy is proportional to the square of the mass. Wouldn't this come from the mass-energy equivalence and thus be mc^2?

Wei Dai's conjecture about black holes being useful as improved entropy dumps is interesting. Black holes or similar dense entities also maximize speed potential and interconnect efficiency, but they are poor as information storage.

It's also possible that by the time a civilization reaches this point of development, it figures out how to do something more interesting such as create new physical universes. John Smart has some interesting speculation on that and how singularity civilizations may eventually compete/cooperate.

I still have issues wrapping my head around the time dilation.