From what we can see of the 4080 leaks the 12GB is on power with a 3090 and on average about 15% faster than a 3080 12GB, or 20% faster than the 3080 10GB. The 4080 16GB is ~50% faster than a 3080 10GB and 72% more expensive.
3080 10GB was $699 MSRP (availability aside)
4080 12GB is $899 MSRP
So an almost 30% increase in MSRP for 2 extra GB of VRAM and a ~20% increase in performance.
It's not even 20% improvement, looking at aggregate benchmarks, 3080 has 53% of the performance of 4090 at 4K.
4080 12GB has only 47% of the CUDA cores of 4090 and they run at the same clock, so you'd expect it to have 47% of its performance.
There's no way that 4080 12GB is going to be 20% faster than 3080. At best they are equal, but more likely that 3080 is going to beat it by a few percents.