S'cuse the expression, but I personally think that's cr*p.
I've been in two E34s now that have powerflex bushings in the subframe - and they've both been as quiet (or at least not noticably different) as the cars without. These arent solid blocks of plastic that just transmit every chassis resonation into the car - they're rubberised just enough to cut most of it out. They are however *hugely* noticable in the rear end stability - made a hell of a difference to how the cars drive.
Whiskychaser -
mate, should make life easier! Where'd you position your jackstands while you were doing this job?