More damage = still reduced to half. Building more damage won't just "work" because all of your damage IS basic attacks. Therefore reducing your attack speed by 20% reduces your damage by 20%. Building more damage still results in a 20% reduction in damage just as building more attack speed still results in a 20% reduction in damage. The best thing to do is ignore these targets, however if it is the ad carry who has this it is pretty hard to ignore it.
Also, 20% more EHP from health or resistances accomplishes exactly the same thing as 20% attack-speed slows, so I'm not sure exactly what point you're trying to make. Building damage, damage, and more damage is the counter to targets with defenses, defenses, and more defenses. Obviously attack speed, attack speed slows, critical strike chance, and armor all multiply and divide damage. You counter dealing 20% less by dealing 20% more.