Syntace has a chart on their website for the P6 Hiflex that indicates whether or not you can shorten the seatpost.
Just a followup here.
I posed this question to the guys at Syntace-US, who forwarded this to the German HQ, apparently there was something lost in translation.
The chart means shows the sizes available from the factory (delivery condition on the chart), and "can be shortened" means that it's not available on that size, but you can shorten to that size from the original length if you want. So the 480mm can be shortened to 300 or 400mm (duh, not sure why they had to tell us which sizes you can shorten to if it's infinite).
So the real answer is, they all can be shortened.