Major Sir Gerald Burrard, The Modern Shotgun, Volume II, The Cartridge, 1955 3rd Revised Edition, p. 154 in reference to modern star crimped paper cases
For all practical purposes any increase in pressure due to the longer cartridge case really does not exist provided the correct powder and shot charges for a nominal 2 1/2 inch cartridge are used.

A summary of Bell's and Armbrust's study is about 1/3 down here
https://docs.google.com/document/d/1ZIo0y746UsSRZIgRuuxwAbZjSBHitO_EanvwLYc-kGA/edit

https://sites.google.com/a/damascusknowledge.com/www/home