My post is going to be way to late to offer any usable input on this issue to the original question, but thought I would, none-the-less, add my input for reference. I own a Reliable 1541S, and have for two or so years. The primary use is in making chaps, chinks, saddlebags and the like. I have made an occasional halter, and some saddle repair. The results are as follows:
Up to 1/4" leather, no problem at all. The machine has more than enough power, and this is generally where I work- below 1/4". On occasions, I will do a project that involves 3/8" (three layers of 9-10oz, for instance) with mixed results. If I split all the leather so there is little or no chance of going beyond this thickness, all is well. If, however, I am in a rush and allow this thickness to grow, I am going to have skipped stitches, and broken needles- again, it is not a power issue, but simple a capacity issue due to needle size and machine engineering (says I).
Overall- a great machine, used within the design limitations, and I would buy it again.
As for me, I am now looking for a second machine for the rare times I need to sew over the 3/8", and I am seriously considering a Cowboy CB3200, so feel free to give me your input.
Hope I have been helpful.