Hi
first of alll i as a amd fan believe crossfire could be more useful regard to your card specifications.
i believe crossfire of high end cards (not the most high ends) such as R(-270 - 270X - 280 - 280X ) with exactly two same cards can increase your performance up to (%70 -%85) but in addition to cards , ability of other parts like mainboard , cpu performance , and ... are also important also i think cross for more than 2 cards its waste of money and it never worth it
therefor i suggest you should change your card because even cross of two r7-260X wont help so much but cross of two R9-270 would be quit much better but as i said your main B performance and specifications are important too. for instance:
990fx mains support @16+@16=32...