2x performance per watt is easily possible imo.

If we talk about per wattage look at the ga102 chip.

I first had a 3070 and now a 3080.

The 3070 was reference. 220w stock.

With an undervolt 925mv and 1905mhz I always had above stock performance while consuming only 130-175w depending on the game. That's up to 40% less. That was also in demanding games. BFV or Far Cry New Dawn both pulled around 135w.

The ga104 chip can get more efficient as well, but not as much as the ga102 chip while you can have a bigger performance uplift.

Why does undervolting work that great this gen?

The factory voltage is way higher than necessary for all ampere gpus.

If nvidia would simply decide to release these gpus with a better factory voltage curve they would be way more efficient.

This too high voltage for ampere makes it easier for the next gen to gain more performance per watt. It also resulted in way too high tdp's for the gpus.

2070 to 3070 went from 175w to 220w.

2080 to 3080 went from 215w to 320w.

The 2080ti which is around a 3070 was at 250w. The efficiency gain looks rather small here. Except you undervolt. The 2080ti can't really get undervolted without losing performance. The 3070 can gain performance while pushing down the wattage massively.