You are right, power supply improvements are asymptotic. It's not just about total efficiency, but also about how the heat is generated and where you can put it. A lot of the newer developments don't do a lot for total efficiency but produce smaller supplies by more efficiency managing heating.

That said, most supplies for sale are based on old, fossilized designs created ~10 years ago, hovering at around 80%. If you spend more you can get the gold, titanium, etc. But then, even those tend to be below the maximum you can get, which is closer to 98% if you have information about your load. You can also design the system to have better efficiency at various loads. Typically half, but could be worth tracking newer CPU/mobo/GPU developments.

The big improvement is you can get the same performance in a smaller space. This either lets you go more efficient (and the difference between 90% and 95% efficient is halfing the amount of heat to get rid of) in the same space or smaller for the same efficiency.