Something doesn't click here. You claim "that we live in an unbelievably computationally expensive universe, and we really don't need to. We could easily be supplied with a far, far grainier simulation and never know the difference"; but how do we know that we do live in a computationally expensive universe if we can't recognize the difference between this and a less computationally expensive universe? Almost by definition anything we can measure (or perhaps more accurately have measured) is a necessary component of the simulation.