TyeRS

Just did an order on the RS3 switching my f80 M3, it'd be a great experience to drive an Audi for the first time. The car seemed perfect, but I realized it doesn't have the Limited slip differential built in. It's strange that all the RS models or even the 4S above have the differentials but the RS3 or TT models do not. Are they intentionally not installing this feature or it's just the Haldex system doesn't allow it?

superswiss

The RS3 and TT RS are built on a very different platform with a very different "Quattro" system. With the Haldex system, the primary driven axle is the front axle and the rear axle is the secondary driven axle. Without getting into another argument, the Haldex system is front biased, so a rear active differential would be kinda pointless. The higher level RS models use a rear biased center differential. Under normal conditions, 60% of the engine torque is sent to the rear, so the rear axle is the primary driven axle. The R8 uses a multi-plate clutch system similar to the Haldex system, but reversed just like Porsche. While the clutch in the Haldex system is on the rear axle and varying torque split from 100f/0r to 50/50, the R8 is the opposite with the clutch on the front axle varying torque from 0f/100r to 50/50. A lot of confusion out there between torque split and torque apportion, so I'm not gonna start another argument here and get into the technical details. There are some lengthy treads which discuss the subject.

The RS3 and TT RS use passive torque vectoring to help reduce the front bias. This system applies the brakes on the inner wheels (front, rear or both) to induce an additional yaw moment to help rotate the car. The higher level RS have this same system, but in addition they get the active torque vectoring rear differential, which makes them rotate a lot more like an RWD car. You can rotate them under power, whereas the RS3 and TT you have to make use of trail braking and lift off oversteer to get it to rotate.