I have a recording of that at home. Glorious!
Our choir director (who is a genius with his own ideas about medieval and Renaissance performance practice) takes it a good deal more up tempo. We also sing the chant all the way through, then after the altos repeat the first phrase of the chant, we launch into the Palestrina.
It is one of those motets that is just as much fun - if not more - to sing as it is to listen to. The parts interweave and respond to each other like partners in a dance.