We now have excellent data regarding turn-of-the-century shell pressures, which, with Dense Smokeless, were very similar to today's
https://docs.google.com/document/d/1F2sQuPm05IE4VWYYnCkvuXmYEzQoWd_SQgaAfUOZEFU/edit

Jon: are you aware of any modern pressure data regarding the pressure increase in using long shells in short 20g or 16g chambers?

This is all that I have found, and it's 110 y/o
“The Long 20 Bore Cartridge”, The Field in Forest & Stream, October 17, 1908
https://books.google.com/books?id=ejQevDPMUIYC&pg=PA630
2 3/4” case with “Schultze” Bulk Smokeless (42 gr = 3 Dr. Eq.)
The standard 2 1/2” case was loaded with 33 gr. “Schultze” = 2.36 Dr. Eq. (about 2 3/8) with 13/16 oz. shot.
36 gr. = 2.57 Dr. Eq. with 15/16 oz. = 4.65 tons = 14,504 psi (using Burrard’s formula) / 20 yd. velocity 1000 fps. That is the post-1924 Belgian 20g proof pressure.
37 gr. = 2.64 Dr. Eq. with 7/8 oz. = 4.54 tons = 14,134 psi / 1040 fps
35 gr. = 2.5 Dr. Eq. with 13/16 oz. = 3.45 tons = 10,472 psi / 1068 fps

It was Progressive Burning DuPont Oval, developed for the 1922 introduction of Western Cartridge Company’s 12g ‘Super-X Field’ 2 3/4” 1 1/4 oz. 3 3/4 Dram Equiv. shell, that enabled the use of large payloads at lower pressures.