Thanks for sharing the video's. I just checked them both out (machine review, and steaming performance video)
e61 groups (designed to emulate a lever group) do run a thermosiphon to keep the group warm. Although typically it's been a different line feeding the water for the espresso, thus the HX flush needed. It appears like they've done the same as my Londinium, where the thermosiphon heating the group is the same water which then brews the coffee. It's a great system/idea with only one drawback. After each shot you need to purge 1-2oz of water to restart the thermosiphon. which if you're doing a cleaning flush will happen anyways. So no real problems there, it's a great system. And great for any novices using the machine, no HX dance to do
But, back to the steaming performance. Now that I see exactly how this machine is plumbed and works (and it's tank size [2 liter] and heater wattage [1400w]), it's steam performance will be analogous to my Londinium (2.3 liter boiler and same 1400watt element). You'll certainly be able to steam with .8bar, but above 1bar makes the steam notably more powerful. I run my Londinium at .95-1.2bar in the winter (uses a pstat, sadly no PID here) and while steaming with the Londinium it's around .95bar, so it gives decent steaming performance.
As for the P500, Mark at WLL said the performance on the P500 drops to about .77bar. You can certainly steam like that, as the video shows, it won't be terrible. Just fine for small amounts of milk, like maciatto's (<3oz), capp's (6oz), or the odd latte (9-12oz). But it will also be a wetter steam (you can see in the video where it sputters), and your milk will pick up more water. So I would also still re-iterate my concern about steaming performance if you do a lot of milk drinks, and plan on running you machine below 1bar. In that case you'll probably appreciate the a double boiler, or running your machine above 1bar (which you may not want to do for the right brew temp, thus wanting a db machine). Certainly if you want to use the machine for latte's or hot chocolates, chai tea, etc..and larger amount of milk, you will appreciate it. If it's just for the occasional cappuccino, you probably wont mind. Or perhaps you don't mind it taking a little longer and a little more water in your milk.
Hopefully this helps. I'm not trying to mix things up on you. I just wanted to help you understand what it will probably be like, so you can make a decision, and hopefully you don't have any unfortunate surprises.