For me RadeonSI was a bit worse experience on R9 270 GPU so far.
1) Xonotic is unable to use 1920x1080 resolution - display shows "no signal" if I try to use this resolution. On other hand 1920x1200 works fine (native display resolution is 2560x1440). But wait, HD5770 can set up 1920x1080 on the very same display. WTF? I guess RadeonSI does something wrong compared to R600 in terms of modesettings?
2) Xonotic seems to have performance issues on some maps. Performance just unstable. Most of time it fits 60FPS (vsync acrive) but suddenly can start lagging. Not sure why, but on some maps in some cases performance can drop down to ~40FPS temporarily (which is a shame for R9 270 @ 1920x1200 @ normal settings, isn't it?). I use recent MESA from oibaf PPA and 3.15rc4 kernel so my setup seems to be quite similar. Any ideas what can screw up performance of R9 270 THAT much?
3) One time GPU has encountered lock-up when using UVD and GPU recovery failed, "as usually". Dear AMD, your "recent" GPU recovery code stinks! I remember some years ago driver was able to recover from GPUs lock-ups. But last couple years or so GPU reset code seems to be insane as it brings GPUs to the very same (faulty) state and usually process getting stuck or loops in cycle after facing the very same GPU fault after "recovery". Needless to say, such "recovery" fails to "recover" anything in most GPU lockups cases I've seen.
Other than that, RadeonSI seems to be quite good thing. And R9 270 on its own just amazing piece of hardware, being both powerful GPU and at same time really cool in idle modes (about 30C or so). Really decent work on power management!