Mickey2

You can assume soft leather is well within what the old straight stitchers do. As long as the material easily goes under the presser foot they usually stitch throught the layers with out struggle. I have sewn flat felled seams on upholstery on my 201, it stitched though it with out hesitating. I reupholstered a bike seat and sewed two layers of fairly sturdy leather and it stitched perfectly.

For heavier threads like top stitch thread and extra strength polyester, you need a larger needle. The most common reason for skipped stitches on these are needle in the wrong. With top stitch thread on heavier canvas you need to up the needle size; that's the most common reason for loops under the fabric when you sort of never get enough upper tension. The needle hole needs to be large enough for the thread, the size of needle shaft matters too: The needle needs to punch a large enough hole and the larger groove along the needle shaft will provide enough space for the thread and it's easier to get enough upper tension.

These machines were made in an age were thread were thicker; linen and cotton thread are thicker, compared to equivalent strenght polyester threads. The cast iron straight stitchers are made to take up to size #20-22 needles. I don't know the finer points of a model 28, but I know a 15, 66 or 201 does this with ease. I know a few cases were they owner swear their 28 and 27 do the best top stitching of all their machines on jeans. At least two of them have capable modern machines on hand as well as other vintage models. I have heard owners who use their 27 (larger than the 28) describe it as sturdier and more solid than a 66 and 201. I know a Pfaff 30 is often mentioned as quite capable with top stitch threads and perhaps a notch stronger than a 15 or 201.

If you plan to do a lot of leather work, push the thickness of the leather beyond jacked and hand bag weights, you might as well go for a more suitable industrial model to begin with. The odd project with soft leather is not too much for an old cast iron straight stitcher.