FWIW: I'm on day 13 with my first PAP device (AirSense 10 Autoset) and a Dreamwear mask. I've noticed a couple things relevant to this thread:
1) I too have noticed a slight and annoying reduction in air flow when sleeping on my side, but mostely when the air pressure was at the minimum of 5 cmH2O. I reset my device to have the minimum pressure be 6 cmH2O and that strangling sensation seemed to reduce to the point that I can sleep with it comfortably. Maybe I'll try a minimum pressure of 7 at some point and see if the reduced air flow issue goes away completely.
2) Leaks around my nose as I moved and tossed in my sleep were also annoying, along with a slight soar spot on the septum of my nose. So, for the last two nights, I've been placing a little bit of lotion at the base of my septum which has made the sore spot go away and surprisingly also significantly reduced the leaks when I move around. I think the lotion lubricates the nasal pillow enough that it can readjust as I move around and maintain a seal.
I've been using a DreamWear for about 3 weeks.
Unlike most, I went FROM a P10 and like the DreamWear better.
Leaking around the nose, especially when changing positions was annoying. I would find myself fiddling with it every time I shifted positions, which meant I was more awake than I needed to be more times than I needed to be.
A couple days ago I started playing with the different size cushions. The sizing card that comes with the thing put me squarely in "medium" and that's what I had been using. After playing around the other night, trying out medium wide, large, and finally the small, it seemed like the small might work better.
I've been using the small for two nights now, and both nights have gone MUCH better. Less leaks (both perception and backed-up by data) and more comfort. It just seems to stay put better and I don't feel the need to mess with it as much.
I sleep on my side almost always, and haven't noticed any reduction in air flow. My low pressure is 10, if that matters.
The only thing I would really like to see different is the exhaust vent. I'm conscious of the stream of air coming out, especially when facing towards my wife, and have to "point" it in an acceptable direction. It's a little bit noisier than the P10, but not much and it doesn't bother me.
Could it be that the perceived reduction in the airflow, when sleeping on the side, depends on the pillow one uses? I mean, a stiffer pillow may be collapsing the side tube more than a fluffy one. Or, when sleeping on the side, if one puts his arm under the cheek / side tube.
I use medium hardness pillow and have not noticed any reduction in the airflow when turning from my back to the side position as long as my head is resting on the pillow. When it's resting on my arm... maybe. I'm not sure.