FUD Zilla tested the mixed CrossFire mode with a Radeon HD 3870 and 3850 graphics card:
Some nice chaps from ATI have informed us that the current Catalyst 8.1 supports a mixed HD3870/HD3850 Crossfire mode. We set about checking these claims as they will no doubt make for some very interesting reading for all owners of HD3800 series cards as well as those who are considering getting them in the near future. The Catalyst driver allowed us to activate Crossfire with an HD3870 and an HD3850 card and we were eagerly awaiting the first test results. It's worth noting that Nvidia doesn't have a comparable feature with its SLI mode.
The results vary based on the game but in some games like Half Life 2 the HD3870/HD3850 gives more than a 50% performance boost.