We have a lot of data that shows chamber reducers "work." That is they launch shot loads at near enough to expected/required and with good enough patterns to be relatively widely used. We can check our assumptions to some extent against that fact. If there were no blow-by and no pressure reduction due to bore volume increase, there would be a velocity increase. A large pressure loss due to either blow-by or bore volume increase would lead to a lower velocity. We may have a small increase or decrease in velocity, so we have to believe that pressure changes are relatively small.
That's exactly what I get out of the available data. More testing may uncover new findings, but absent that anything more is speculation (and sometimes speculation is later proved to be right, but we're not there yet).
