We started shipping PIR KumoSensors with firmware that allows sensitivity adjustment today (2015/11/6). When you receive your new PIR sensor (will show as "rev 8F" in search result), you will be able to access the sensitivity slider from the Web interface at https://mytaglist.com/eth.
With this new feature, you can reduce sensitivity if you are getting false alarms due to very short movement lasting less than a second. We tested this feature on two customer returned PIR sensors that were prone to produce false alarms. With sensitivity set to 50%, the false alarm completely went away, while each detects normal human movement every time.