I've sewn thick leather seams with my 15-91 as well as my 201. If I still owned a Featherweight I would have used it to sew leather. The leather I used was a full 1/4" thick when you put 2 pieces of it together to sew a seam, and I had no trouble whatsoever. Click on my blog below, go to the Quilting page, and scroll down to the 7th photo... enlarge it and you'll see the pile of thick leather sitting next to my 15-91 before I cut it up to upholster a chiropractor's table.
If you use the right needle you won't have any trouble. A leather needle will cut through leather like it's butter. The reason I say you should be able to sew leather with your featherweight is that the inards are pretty much the same as the 15-91 or 201... just shorter in places.
A featherweight is not a wimpy machine, you'll be fine sewing leather if you use leather needles.