If you're really THAT worried about performance, buy a used 290x from a 100% positive feedback seller on Ebay. That's what I'm doing in a few months when I upgrade my 270x. The 270x is a fine card, btw. The 280x is also a fine card.
What games do you want to play that your worried about a 280x not being enough? Are you gaming at 2560x1440 or something? That card can run anything at halfway-decent settings
No, just on a 1080p tv. I feel like it's going to fall way behind once stuff like Witcher 3 comes out. I haven't even built my pc yet, but I'm on a tight budget.
30% better is 30% better. If you can't run Witcher 3 on a 280x, you probably couldn't run it on a 290x. There's no magical cutoff point like some people would like you to believe. If you're getting 30fps in a game, and someone else has a card that's ~30% faster, they're only getting 40.