Seems to me, the lower the pressure, the easier it is to breathe. My Autoset is set to 5-15, and when I look at my graphs, I see the machine adjusting itself constantly. The thing that I like the most is that when i'm having problems the S9 adjusts the pressure to remedy the problem. When i'm sleeping/breathing comfortably, the pressure comes down. My actual pressures range from 6 to about 9.
I started with the Resmed Escape and did poorly with the constant pressure. With the adjustable pressure my AHI has dropped down to less than 1 pretty much every day. The proof is in the data.
I also realize that my improvements are also due to experience and better fitting mask.