so I got my Zephyr HxM and it works perfectly,
I didn't test yet the cadence report for running/walking.
I currently use the Zephyr + an external Bluetooth GPS (GlobalSat BT-338, I got cheap from ebay),
and I'am able to use both for 4h30 and the battery goes from 99% to 44 %.
Which means that if battery life is linear !, then I could use it for around 8h00 without problem, and that's with the original battery not even the extended one (which has around the double in capacity ... and size !).
I made a similar test using the internal GPS and the Zephyr and for a workout of around 2h00 the battery went from 89% to 45%. so again if battery is linear it would last around 4h in that configuration.
So given that the BT-338 gps receiver is more precise and can be used for around 17h, I will use it for long workout.