How big an improvement in eMate performance (not storage, which is an obvious enough improvement) with one of these installed? Apparently the width of the system bus is thus doubled, making the machine more responsive. But how much more responsive? Anyone out there experienced the before and after scenario at first hand?

The eMate is the only Newton you can do this with. Presumably it was engineered into it for the sake of the 'business' model which never made it to production.