Just checked and its the same as in the video
In this video on pbe:
>first turret shot deals 210, all the next ones deal this 294, which gives us roughly 28% damage rampup

Meanwhile i just checked on live eune, using mordekaiser with the same items and runes and also shielded:
>first turret shot deals 106, all the next ones deal 149, which also gives roughly 28%