I've made a fair number of predictions about things I need to do on PredictionBook, and I don't think it has had much any effect on my motivation. Boosting your productivity might be possible if you make optimistic predictions if you are strongly motivated to be well calibrated.

Another possible use of PredictionBook for motivation is getting a more objective view on whether you might complete a task by a certain date. If others think you are overconfident, then you could put in place additional things to ensure you complete the task.