Active Steering can exploit extreme setups to make a driver much faster because it helps them 'surf' the limit of grip.
Active Steering doesn't add grip but it does adapt the steering angle when you are 'surfing' on a very slight slide. You can see this in Photo Mode during a replay. In GT6 this slight slide is the fastest way to corner. (Original GT's manual explains this well.)
Customising a setup to exploit this 'surfing' is what all the top TT setups seek to achieve. But the driver has to deliver the steering inputs, timing them with precise amounts of brake and gas. Only the 'platinum' drivers can perfect this. (TRL members for example.)
Active Steering is a performance boost because it adjusts the steering more precisely than almost any human can do it, when the car is sliding. Normal steering is calculated for all DS3 users when we push 100% left or right, so it's probably similar code to how that works. DS3 steering is not always the 'ideal' angle and it doesn't handle oversteer situations. Active Steering is probably not perfect but it does help driving in that difficult but very fast 'sweet spot' of oversteer.
Active Stability Management applies brakes on specific wheels to stop the car spinning around. That is slower because it stops the car 'surfing' the limit of grip
and scrubs off speed. This is a legitimate newbie 'crutch' as it keeps people on the track without providing a performance boost to good drivers. Same story with Traction Control and (arguably) ABS. WRS allow these (along with many other groups) because they keep people on the track without giving an advantage to drivers who are already fast.
An aggressive setup that makes the car rotate a lot would be near impossible for a human to drive perfectly. Immortal at Ascari with FXX is a masterclass in what is possible. (Along with heavily worn and limited engine to reduce high rpm instability, probably other setup tricks as well!) Extreme setups with Active Steering enable a level of performance beyond an accomplished player, especially over a race distance, so it is normally banned in authentic race series.
I remember when SRF was sometimes available or even forced on in Seasonal Events. It is sometimes forced on in Quick Matches. Road cars at Indianpolise road course or Group C Le Mans at La Sarthe this year for example. Experienced players don't like this because it is so unnatural while making you faster than the standards physics. It has no place in 'the real driving simulator'.
Active Steering, when dialled into a specialist setup, is a big advantage and as unnatural as SRF. I wish it was banned from Seasonal Events!
Then again, DS3 drivers like me are not playing the game 'properly' if we decide Active Steering users are not playing the game 'properly'. So I would have to be banned from competing in these events as well because the game is doing most of the steering for me, too.