I wrote an article for "Import Car" earlier this year on MB alignment issues. Half way through writing it the spreader bar issue came up on this forum. It bothered me terribly as I had known of it but never used it over the thirty years I have aligned cars (most of them MBs). I purchased the spreader bar just before the article went to print and from the few measurements made for the article and from many since, I get around 1mm toe change with it. This is fairly insignificant to one who is using tire wear as a significant input to setting specs.
For anyone interested the article can be viewed here:
http://www.continentalimports.com/ser_ic4232.html