The thing to remember about anti-roll bars is that the stiffer you make one end of the car, the more load the outside wheel on that end has to carry in the corner, and hence that end reaches the limit of adhesion sooner than it would otherwise. So putting a stiff ARB on the rear means that in a hard corner the rear tires will reach their limit of adhesion sooner than it would otherwise, and consequently the rear ARB induces oversteer (or reduces understeer). Conversely, stiffening the front increases under-steer (or reduces oversteeer). HOWEVER - the devil is in the details, as this rule of thumb may be trumped by the particular geometry of the car's suspension. For example, with the BMW 3-series it has been found that stiffening the front actually reduces understeer, which is counter to the general rule. The reason for this behavior is the trailing arm suspension geometry in that car - as the springs compress in the turn the outside wheel gains positive camber, which means the tire rolls onto the sidewall and it loses some grip. So stiffening the front with a fat ARB results in less camber change and the front end gets better grip, which more than makes up for the extra load that the front has to carry.
I have not seen any discussions on whether the S5 behaves this way, but I doubt it. From my own experience at the track both with and without the STaSIS rear ARB - it definitely reduces the car's inherent understeer, in line with what you would expect from theory. If the H&R kit includes a stiffer front ARB than stock, then I would hope that the H&R rear ARB is stiffer than the STaSIS. But it is very difficult to analyze this without actually trying out the various set ups - manufacturers may tell you the outside diameter of their product, but they typically don't tell you the wall thickness.
Interested in reviews on the H&R rear...anyone...?
