How does one configure Radeon video cards when using the open source Radeon driver - power profile, vsync, etc?

Why I try the widely documented solution (against overheating) that worked for me under LMDE (confirmed with kernels up to 3.12.6), I get the following error:

$ sudo cat /sys/class/drm/card0/device/power_profile
default
$ sudo sh -c "echo mid > /sys/class/drm/card0/device/power_profile"
sh: echo: I/O error
Exit 1

And when I try suggestions from Arch's ATI wiki my modifications are simply ignored:

$ sudo cat /sys/class/drm/card0/device/power_dpm_force_performance_level
auto
$ sudo sh -c "echo high> /sys/class/drm/card0/device/power_dpm_force_performance_level"
$ sudo cat /sys/class/drm/card0/device/power_dpm_force_performance_level
auto

Is this something Ubuntu specific, or something introduced with the 3.13 version of the Radeon driver? I'm encountering this on 2 laptops, one with a Radeon HD6290 (integrated GPU), the other with a discrete RV710 card. The RV710 needs a specific power setting to prevent overheating under LMDE, fortunately it doesn't seem to overheat with the Ubuntu default setting.