According to Computerbase, GC P-core consumes 21W at 4.9Ghz :
https://www.computerbase.de/2021-11/intel-core-i9-12900k-i7-12700k-i5-12600k-test/2/
They also found that E-Core under full load consumes between 5 and 6 watts.
So if they opted to create a monolithic 16 P-core part, it would had to be clocked much lower (my guess ~11 to 15%) to keep it under the same 241W limit. 12900K with E cores is about 30% faster than without E cores (see
here). Since Ryzens gain between 65 and 75% going from 8 to 16 cores in MT scenarios (average, see
here for 7700X/7950X and 5800X/5950X), my guess is that 16 P core part (with ~10% clock regression and 241W limit) would have performed around ~18-21% faster than 12900K (stock) in MT.
Intel actually did good with 13900K, as they claim to get "up to" 41% better MT performance vs 12900K which is around 17-19% better than a hypothetical 16 P core part based on Golden Cove. They will most likely still lose the MT crown but they are going to be much closer to 7950X versus the hypothetical 16 P part.