Jovial, did you figure out the solution to minimizing the pucker on the backside/bottom stitch in practice? If yes, can you attach pics and explain what adjustments you had to make?
My top stitches look amazing, but the bottom stitches are flat (instead of having a nice curved taper on each terminal of the stitch). The holes in the backside are puckered out because of the stab of the needle. Are these back stitch characteristics just inherent to sewing leather on a machine?
Here's an example of my ugly back stitches... http://cl.ly/image/333s1N0Y3E0K
Ugly back stitching is the most frustrating aspect of sewing leather on a machine, because it complicates everything. When I'm putting together a pattern I always have to plan for how I'm going to deal with that ugly bottom stitch! Sometimes I want to sew with the pretty side down (sueded part up), when I'm trying to sew a pocket to the inside of a bag — but I know if I do so I will be risking having an ugly bottom stitch on the outside of my bag!
Hand sewing has crossed my mind, but it's very inefficient. It is the ultimate solution to having full control over stitch quality though...
By the way, I sew on a Juki 563 (unison feed).