We've wandered into choke rather than forcing cones, but a study by Neil Winston comparing shot speed and choke constriction
http://www.claytargettesting.com/study2/Study2.2.pdf
Full choke is 25-50 fps faster than cylinder at 4 FEET

Neil did not measure, nor can I find 40 YARD data, but the difference would be very small

Note SAAMI Shotshell ballistics are reported through a test barrel with .031 constriction