If bitcoin mining becomes more efficient, miners run more of them, the hashrate increases, the difficulty goes up, and we reach a new equilibrium - with the same overall amount of value being created, and therefore assuming efficient market, the same amount of energy being expended. The blockchain is worth the same whatever the hashrate (at least if we're just talking about more efficient miners - if more effort is being put into mining then the blockchain becomes more secure, but if someone makes a more efficient miner then that same miner also makes attacks easier).

In contrast when gold mining gets more efficient, either we expend less energy, or we produce more industrially-useful gold. Admittedly in the limit where people are just buying gold and putting it on the shelf I guess the same considerations as bitcoin apply? Hmm.

The value of gold mining is gold, which has various personal and technical uses. The value of Bitcoin mining is to support the blockchain. The value of which is, honestly, still unknown, but so far appears mainly to be transferring money.

So if we're measuring ecological efficiency of Bitcoin, it'd be something like cents or watt-hours or grams CO2 per financial transaction. I've never seen a rigorous comparison, but I wouldn't be at all surprised if Visa and Paypal are more ecologically efficient than Bitcoin.