> Is bitcoin sufficiently trustworthy without proof of work? I'm not a bitcoin expert, but my understanding was that this is an integral part of the trust model. If so then we're just back to talking about inefficiencies.

Yes, that's exactly the point. Bitcoin is insanely inefficient because PoW is insanely inefficient. Unlike other "inefficiencies" where there is a logical relationship between "energy spent" and "useful work accomplished", with PoW there is no such relationship since the energy isn't spent doing useful work (i.e. processing transactions). Yes, more power into the network makes it "more secure" by making it "more expensive" to perform a 51% attack. The increased expense is distributed across the network in the form of increased difficulty. If a 51% attack was already implausible (which most bitcoin enthusiasts would say is true), then making it even more expensive doesn't actually do anything useful.