How does workgroup size impact the kernel performance ?

On a GPU the workgroup size often dictates the occupancy of the compute units. With FPGAs the workgroup size typically just dictates how many resources will be necessary to create things like the barrier logic for example. Remember that on the FPGA the compute unit is a flexible piece of hardware that is tailored to your kernel so it's not a fixed resource that you are trying to achieve 100% occupancy on. So for algorithms that work well in an FPGA the high occupancy rate "just happens" due to the compiler tailoring the compute unit to your kernel.