The answer is a bit complicated because it depends on the games that you play and how well they are written for multi-core processors. Some properly written games run up to 50% faster with the AMD FX processors but not all. With poorly written games the full potential of the R9 280X will not be used.
The R9 280X is probably a good match for the FX-6300, IMO. I'm sure there will be online test results with the FX-6300 but you may need to wait a bit for them.