Not much http://www.tomshardware.com/reviews/radeon-hd-6970-rade...
"We explicitly talked to AMD about CrossFire scaling after its press briefing. According to the company, it didn’t make any changes to the Radeon HD 6800 series that would have improved its CrossFire scaling versus the Radeon HD 5870, as many folks suggested in our 6800-series launch coverage. It did, however, incorporate driver-based enhancements that augment CrossFire performance in the 6900 series. This seems to be reflected in the results here.
Now AMD says that the 6800s will get the CrossFire-oriented improvements in a future driver update. For now, though, 5800-series and 6800-series cards should scale comparably, with the 6900s doing even better."
So thing will get better for HD 58xx series with future drivers