Should giving a negative torque make it turn slower?

I'm spinning a rigidbody using AddRelativeTorque, and on a keypress want to instantly reverse the spinning direction, which I do by turning the positive "rotationSpeed" integer into a negative. But this doesn't spin as fas as the positive number - is this correct?!

So in a FixedUpdate I do:

 GetComponent.<Rigidbody>().AddRelativeTorque (0,rotationSpeed,0, ForceMode.Impulse);

...then on a keypress reverse the rotationSpeed by doing:

 rotationSpeed = -rotationSpeed;

My question is, how can I spin the object in reverse at the same rate?