This is a fascinating way of looking at it.

My first thought was to reply, "Yes, most worlds may need to be pruned a la Hanson's mangled worlds, but that doesn't mean you can end up with a single global world without violating Special Relativity, linearity, unitarity, continuity, CPT invariance, etc."

But on second thought this seems to be arguing even further, for the sort of deep revolution in QM that Scott wants - a reformulation that would nakedly expose the computational limits, and make the ontology no more extravagant than the fastest computation it can manage within a single world's quantum computer. So this would have to reduce the proliferation of worlds to sub-exponential, if I understand it correctly, based on the strange reasoning that if we can't do exponential computations in one world then this should be nakedly revealed in a sub-exponential global universe.

But you still cannot end up with a single world, for all the reasons already given - and quantum computers do not seem to be merely as powerful as classical computers, they do speed things up. So that argues that the ontology should be more than polynomial, even if sub-truly-exponential.