The only way I could ever see a sway bar affecting ride quality in the way explained above is MAYBE if you hit a bump with only the passenger side wheels and as a result of the stiffer swaybar linking between the driver and passenger side, the driver feels the bump more than normal. But I don't see this as really being an issue, even if it works that way (which I am honestly not sure about, I'm not that well-versed in this area). It reduces roll by reducing flex, but I don't think that flex reduction really influences the transmission of road shocks from one corner to the other. You hit a road imperfection with your passenger side tire, it extends/compresses then rebounds. It doesn't transmit to the other side... (at least I don't think so. It doesn't make sense to me that it would.. correct me if I'm wrong).
The weight difference between the M3 sway and the 135i sway is negligible because they are both hollow, and even though the M3 one is indeed thicker, since it's hollow you're not really adding much mass to the bar. But even if it wasn't, it really wouldn't matter. The H&R bar is indeed solid and not hollow, and even that one has no detrimental effects to handling.
The only negative effect I can even forsee is if the front is much more stiff than the rear, this can increase understeer. For fixing this, I really like this web page:
http://www.nyracer.com/overunder.htm