No, but I've used a Saivo (3-person) and Jannu (2-person) in winter with two people, and the Saivo in all seasons, and have a few observations:
First, in winter, it's not unreasonable to subtract one off the official "number of people" rating of the tent due to larger sleeping bags, extra clothing, etc. I've found the Saivo to be just right for two people in winter, and bet it would be a tight fit for three even in good weather, though the dual vestibules make a huge difference.
The Jannu was a tight squeeze for two in winter, though doable.
In both cases, filling the tents to their max rating in foul weather would make the experience less comfortable.
Both tents seem to develop a fair amount of condensation on their floors when pitched on snow, which gets worse the more people are inside of it, and the more active they are. Wet weather compounds the condensation issue (not unique to these tents, obviously), and the more people you have in there, the more you'll add to the moisture load.
The Nammatj shares a feature of the Jannu: one vestibule, plus a sloping rear portion of the tent. Because of that, the flexibility goes down, and with more than one person in the tent, it's likely that sleeping bags will end up touching the rear sloping wall. Hilleberg suggests draping a shell jacket over the foot of your sleeping bag to compensate.
Because of the vestibules at both ends, I've found the Saivo more flexible and comfortable for extreme weather, and I'm betting the same would be true of the Keron versus the Nammatj. Both of the latter would be more flexible in severe weather with the GT extended vestibules, but of course that adds some weight.
Beyond those issues, these are superb tents for bad weather; stable, waterproof, and reliable - also easy to pitch, and because of the way the inner is attached to the outer tent, able to be pitched without getting the inside wet or snowy.
What, specifically, are you hoping to learn? What conditions and types of trips do you hope to use the Hillebergs for?