Not sure what normal would be. I try watch the "slip guage" when I put it on a high setting. I've seen it go up to around 2% at WOT on the extreme setting, then the box starts to defuel. I checked the records once and I saw it said like 7% or something. Now I have a manual, so I'm not sure if that was just from throttle/clutching at the same time or if the clutch was actually slipping.
BTW, when I first got the box, I never really noticed it defuel. Now on the extreme setting, I can deffinatly feel it dropping the power to save the clutch. I hardly ever use extreme, I'm no racer, I just have to smoke people sometimes.