I've done Berlin-Munich by car many times, maybe I managed that under 5 hours once, in the dead of night. But the usual is a couple (5 or so) construction zones which totally kill your average. 6.5 hours is fairly typical if you need to gas up once and take appropriate rests.
5 hours seems optimistic by car, 4.5 by train as well given DB, German railways, track record. The big issue in Hermany is that supposed higj speed trains have simply too many stops. As a result the ICEs last generation emphasizes acceleration instead of speed.
France has it some how easier with dedicated higj speed tracks and basically no stops. Just as example, the one stop before Munichs central station is adding almost 10 minutes.
Berlin - Munich: By car 5.5 hours plus breaks. With the ICE 4.5 hours.