1 - No, not if you're going to use an R9-280X
2 - No, not if you're going to use an R9-280X
3 - Yes, it will bottleneck an R9-280X
If you're going to get an R9-280X, forget the A8-5600K and the FM2 motherboard and just buy and FX-6300 with an ASRock 970 Pro3 motherboard. It will be cheaper and more powerful. If you're not going to use the APU's graphics, there's no point in buying an APU.