I have a Radeon R9 285 (MSI) and am looking to upgrade by adding a second card in Crossfire. I've spent some time reading documentation and searching forums and haven't seen it stated exactly what it's compatible with, and the chart referenced by much of the documentation (http://support.amd.com/en-us/kb-articles/Pages/Crossfire-Chart.aspx ) doesn't include the 285 or any of the 300 series or the Fury cards.
1. It seems likely and some forum posts but no AMD documentation says that a 285 will work with a 380 in Crossfire. Would any of the other (higher-end) GCN 1.2 cards work too (discounting the unconfirmed possibility of nearly any two cards working together under DirectX 12)? Whether the extra memory would be used is my primary concern, as the clock speed differences are minimal.
2. It seems that the two main differences between the 285 and 380 are the clock speeds and the option of having 4GB memory. Would a 4GB 380 in Crossfire with a 285 just be reduced to the clock speeds and memory of the 285? Would it make use of the extra memory but not the clock speeds? The inverse? Several articles mention that compatibility but none link to confirmation by AMD or test that combination