Without knowing how often your RLE is hitting the max length of 16, but assuming it was often, a further optimization could be using one bit as a flag and to si

Without knowing how often your RLE is hitting the max length of 16, but assuming it was often, a further optimization could be using one bit as a flag and to signal that the following block is pixel is either a small sequence of 1-8 pixels, or a large sequence of a multiple of 8 pixels (ie. 1 = 8x1, 2 = 8x2, 3 = 8x3). This lets you compress up to 64 pixels into the space of no more than 2.