During the AMA with AMD at Tomshardware, the AMD guy said that they will be releasing a fully stable (which I assume to be non-beta) and good driver for the AMD-spec SteamMachines. It is just that Nvidia has a head-start with Valve.
I have finished Metro LL on maxed settings with the R9-290X with CCC A.I. on; and enhanced AA using edge-detect. Worked smoothly but I can only surmise that it could've been even better and more smooth if AMD had a proper linux driver.
Lastly, watching --odgt and --odgc while playing games or running furmark; I can see the down-throttling quite a bit. With Serious Sam BFE I saw it reach below 800MHz!!! Not as much in Metro LL which I find peculiar as graphics fidelity with Metro LL is significantly better than Serious Sam. Perhaps Serious Sam uses the card like Furmark which doesn't look spectacular but does heat up the card. It did get hot in the room I was playing in though during that day due to warm wheather (late spring here down under).
So that said I have decided to upgrade the cooling; I'm likely to water cool it during the Christmas break or if a good dual-slot aftermarket cooler comes out for sale with good performance and acoustics, I may go down that path as well.