The ABS in GT5 is totally different than the actual ABS in real life. there is no actual mechanism of ABS simulated in GT5, ABS in GT5 is nothing but a glorified brake assists that will apply full brake force without locking up, the numbers of ABS strength only applies to how much the steering can still be used while braking hard.
Try to drive a real car with ABS, brake as hard as you can while turning into a corner, and feel the brake pedal ... when ABS system detect skids/ tire lock up, it will kick in, you will feel pulsation on the brake pedal and the brake distance would lengthen considerably when the wheel is also turning. Try that in GT5, the car will lose speed so quickly, you won't believe it can be done in real life. Then do so in GT5 WITHOUT ABS, try to do threshold braking, you will feel that when the tires are about to lock up, the car will lose balance .. I recommend to try 4/2 brake balance for a start, if it understeers, use 5/3, then gradually increase to 6/4, 7/5 till you find the sweet spot. With DS3, I usually use 5/3 or 6/4, if using racing tire, I can increase to 7/5
Another notes, when going at high speed, eg above 200kmh, without ABS, we can brake fully without locking, then we need to gradually decrease brake force as the car losing it's speed, much like in real life, practice and practice is the key