I looked at this very effect once based on the computed air pressure in the barrel of a springer at the moment of pellet release. I wrote a small time stepping program that computed the flow over the back of a pellet. There is a change in velocity but it is nominal, a fraction of 1ft/sec. It may be higher for a PCP but it will tend to vary depending on barrel length, pressure etc.. If the effect is large it will not be good for dispersion as it could affect pellet yaw effects.
I have not looked at small arms bullets but, unless the barrel was very short, I would be surprised if the increase was as large as stated since it would suggest a very inefficient charge being used with a very late all burnt position. I am not saying it is wrong, just surprising. It would also be bad for bullet dispersion as mentioned above.
The use of silencers or suppressors would increase the effect as the gas would still be constrained after leaving the barrel.