The perches in question are the mounts at the top of the front coil springs. The ones from the B5 generation A4 fit the B6 and B7, but have the effect of lowering the ride height by more just what lowering springs alone would achieve.
An anti-roll bar (ARB) is a metal bar sitting cross ways, connecting front and left suspension assemblies. As the name suggests, they reduce the car's innate desire to roll in corners. The thicker the anti-roll bar, the more roll stiffness is achieved. By a curious quirk of geometry, increasing rear roll-stiffness on its own has the effect of reducing understeer and making the front of the car more responsive to steering inputs. Increasing front roll stiffness on its own has the opposite effect, and increases understeer.
The rear anti-roll bar on standard A4s is 16mm thick, The S-line is 18mm, the S4 20mm and the RS4 22mm. Those increases might not sound much but due to another quirk of geometry, a 22mm bar has almost double the roll stiffness of a 16mm bar.
If you fit an RS4 rear anti-roll bar, you will vastly improve your car's handling without impacting on the ride. The parts cost slighlty under £100, and it takes about two hours to fit.