A stiffer rear bar isn't going to reduce understeer, it's gonna reduce roll oversteer instead.

If you make it and the front bar stiff enough, you can get the W115 problem -- it will corner hard enough to force the front outside tire off the rim! This problem was "cured" in later models by reducing the sway bar stiffness some, to allow more body roll. This unloads the front tire and causes the rear to lift and start to slide, permitting the driver to retain control and recover gracefully (as in on the road instead of in the ditch or up alongside a tree or something). It's not as pronounced in the multilink rear end since you don't get the camber/toe changes you do with the semi-trailing arms, but it's still there.

I've been told the multi-link rear end is less forgiving than the semi-trailing one, but I've never driven any of my cars hard enough to find out -- not possible with the 220D in any case!

I've not noticed any understeer to speak of in the W124 (nearly identical to the W201 chassis suspension), so I'm not sure why you would want to "reduce" it.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!