It may not be ready for us to use mantle but I didn't see anything about it not being supported in the "alpha release" posts unless I just missed it. We'll see.
The 7000s series has a hardware level bug in it they we have to work around, so it's not supported yet in the alpha. The 270 might have the same bug, I'm not familiar enough with the product name vs GPU Version. (It's a bug in GCN 1.0)