I'm not trying to make my tele produce the same bokeh as the 135L. If I could, there would be no place for the 135L in my bag. I'm trying to see if turning off IS would make the bokeh improve somewhat on my zoom. And if IS does have an impact on bokeh quality (a hypothesis), why bother having it if I consistently use at least 1/125 shutter speed? And people say they want a new 135L with IS, if the hypothesis stands, wouldn't that mean the bokeh quality won't be the same as the original 135L?
Well, try it, and let us know. I do not expect too much difference. On the other hand, a lens having IS would have a different design, and this might
affect the bokeh visibly.
IMO, the main reason for not so great bokeh with some lenses is because they were optimized for sharpness. This includes some of the 70-200 lenses, the Sigma 35, and the 18-35, and maybe others. There is a well known and easy to google Zeiss document about bokeh and it explains how over- or under- corrected aberrations affect bokeh.