Just like any farm, some pearls will have thicker nacre others won't. Nobody knows until the pearl has been drilled, and Josh's pearls don't have thicker nacre than another farmer's. Also, Tahitian pearl nacre is opaque, so 0.8 vs 1.8 will not make any difference in the depth of color. Color (at least overall color of harvest) is more influenced by the area or atoll where the pearls are produced. Some atolls consistently produce a lot more grays, whereas others, such as Rikitea, are known for producing vibrant colors and darker pearls.
The size of pearl you go with for a leather piece really depends on the person. I've seen people wear smaller (in the 10 mm range) pearls fine, especially when worn very close to the neck. When worn lower, I feel the larger pearl the better. I personally wear a 15 mm pearl.