This "Anonymous" character is great at guessing but not great at guessing correctly. The FX-6300 will not bottleneck the R9-280X in any manner that you will be able to detect. Bottlenecking is the new Boogeyman. It's funny how we never used to worry about it before but now it's as overused a word as terrorism. In the strictest sense, bottlenecking occurs with ALL CPUs when using top-end cards. In the real sense, bottlenecking means that the CPU will drag your video card down into unplayable frame rates. The FX-6300 will not do that to any GPU. Hell, I run an FX-8350 and I have two of those cards crossfired. No bottleneck here. You're fine and remember, bottleneck and boogeyman have something in common. They're both greatly exaggerated.
Remember, if someone is posting as "Anonymous" it means that they don't even know how to get a tomshardware account or are too lazy to create one. Always take that into account.