The underside of the engine bay of a street car is a high pressure area. Remember
the front subframe ist the lowest part of our cars which slows the air flow. Even more
the air flowing though the engine bay (which entered via the radiator) has to leave
it somewhere. The flanks/sidewalls of the car are low pressure areas. This is common
knowledge and can easily be read in a subject book about aerodynamics.
So the air flow has a V-shape like pattern towards the wheel wells. The RS3/TT-RS
vent directions this flow to the brake discs (or to the dustshields if still in place).
Still common knowledge.
From my understanding the RS3/TT-RS vent can not ensure it has a full effect if the
directed air flow is blocked by the dust shield.
I never had any issues in the wet or even in the winter with lots of salt on the road
if you fear this.
At least if you have slotted, drilled or J-hooked rotors I don't expect running without
dust shields to be problematic at all.