This video shows the Jello Effect when ETC mode is used. I used the 14-140mm @ 35mm plus the 3.9x 720p ETC and the 14-140mm @ 140mm to give the same equivalent field of view.

I filmed passing trucks at the same speed with 1/250th shutter speed. I could not tell any difference in the angle of distortion between the two clips. A lot people think that rolling shutters only affect the video if the camera is moving. It will actually slant vertical lines if the subject is moving fast enough horizontally even if the camera is still.

I do not believe that using ETC mode over just the lens has any affect on the rolling shutter jello effect. Theoretically sensor readout speed is the only thing that could affect it. Using ETC mode should reduce the number of pixels that the camera reads. However, that does not appear to help at all. Instead it appears to read the entire sensor in the same time that it takes to read it without using ETC. Maybe it is just throwing out the data the ETC mode doesn't need?