I can't speak for Kaplan, but I found the GMATPrep software (the OG thing you're referring to) to be a fairly accurate predictor on the verbal side. I scored a little better in real life, but I wouldn't bank on that. It was probably just luck.
On the other hand, I thought the real quantitative section was much more difficult than GMATPrep, but my score was in the same ballpark. I feel like they ask tougher questions but score more leniently on the real GMAT quantitative section.