They'll still come off of a pusher. We've got a member of our RC club that builds little F-15-ish jet models powered by way-fast pusher motors; the word on prop savers is "Oh god no, you don't want to do that!". This is based, I gather, on real-world experience involving chewed-up foam.
OTOH, if you run the motor up to speed and flick the model 90 degrees in pitch and the prop doesn't come off, you're probably fine at that speed. At least until the rubber band gets tired.