for Christmas, I made a bag from australian waxed canvas/leather, and another all leather. I originally started with a hide from BMW upholstery- really nice feel. With my 201 and a walking foot(I also used teflon foot and roller) I could sew 2 thicknesses of the upholstery leather. This hide was advertised at 3oz weight, so 6oz total. I could NOT sew anything heavier, and I wasn't really pleased with the 201.
Moved to a heavy Morse SS- it did better than the 201, but still struggled.
So I moved on to the 130 with 1.5 amp motor. It was able to handle three thicknesses of the BMW hide if going slow. It was able to handle 5 thicknesses of the waxed canvas. Where I had to really work at it was seam intersections- and I changed up the pattern so that I didn't have more than 2 leather/2 canvas.
I used MOLLE webbing from the military store for strapping and could sew 2 thicknesses of that combined with 2 of canvas. I couldn't sew 2 webbing and leather.
I used a #16 or #20 leather needle and 92 weight thread(top and bottom).
First shot shows 4 layers of canvas/1 leather. Second shows 2 layers of 2oz orange leather and 4 canvas. I was able to topstitch pretty well for these.
Like anything else, upholstery leather comes in good and crappy quality. Poor leather with sealers/wax on it will be hard to do on a 15 clone.