macaman
What's IBT?
I think I kind of get what you're asking. Understand that not all computer based tests are CATs. Here's my take on the practice gmat's I've taken.
GMATPrep - the gold standard. Do not waste it on a whim.
Princeton Review online test - math is too easy, but incorrect answers punish your score. verbal is on par. overall scoring is similar to actual gmat.
Manhattan non CAT - very difficult, expecially the quant. scores are lower than actual gmat, but not by much, I'd say no more than 30pts
Manhattan CAT - easier and scoring is a bit inflated.
Kaplan paper - ridiculous