1. Yes, as long as resource usage of the kernel (register allocation and use of local memory) allows. Otherwise the workgroup size might be 128 or even 64.
2. You can simplify this and say that 5120 wavefronts are split across 20 SIMDs, which produces the answer of 256 wavefronts per SIMD. The hardware might do a slightly uneven spread though (e.g. if the kernel follows immediately after another kernel). In truth this unevenness doesn't matter.
As long as there are more than 2 wavefronts per SIMD you are getting decent performance. AMD recommends at least 3 wavefronts for ALU-heavy code. And at least 5 for memory-heavy code.