Improvements in PoW come in a form of advancements in energy-efficient computation
You mean improve on how to waste energy more efficiently?
I think he means greater actual security with the same security cost.
People keep making the claim that PoS offers lower cost.
This is only true (as far as I know) if the currency issuance
is lower, due to competition.
But, even if we can lower security cost, the question
remains if PoS is a fair system, as it tends to "ossify nobility"
as Vitalik would say, and makes the rich richer, eventually
culminating in the hunger games
It may be true PoS offers some higher security though,
which is why I'd be interested in this:
https://en.bitcoin.it/wiki/Proof_of_Stake#Meni.27s_implementation