Hey Vincent, I will try to find some photos that I took with the lens stopped down, I usually don't shoot it that way, but I think I have a couple photos lying around here somewhere....I will post them when I can...
As far as the 15mm goes, if I am using my m6 then I use the internal meter and I will usually open it up a half stop, it depends on the picture obviously, but I tend to open it up a half stop of whatever my meter is telling me when I use the 15mm. If I am using it on my m3/m2 then I use a hand held meter (sekonic 358) and I just go with it, I don't tend to meter a lot when I am shooting, just a couple quick checks and adjust as I shoot...
As far as composing is concerned, the more you use the lens and experiement with it when you first get it, you will learn how to use it and how NOT to use it...composing can be really easy with it , because after a while, you start to see 15mm shots and the viewfinder is just there for a quick check.....
I can shoot pretty fast with 15mm...the focusing is easy...I used a rollei 35 for a couple of years as my only camera and I learned how to adjust the focusing and aperture on that and now when I use the 15mm, it is so easy to do.
hope that helps...cheers, michael