Created attachment 122105 [details]
dmesg output

You are right - I notice:
[    3.055513] [drm] Loading BARTS Microcode
[    3.055579] [drm] Internal thermal controller with fan control
[    3.061488] [drm] radeon: dpm initialized

This is interesting - as the fan sure works harder under the radeon driver than
the fglrx - and you can feel the extra heat coming out the back of the card.

Is there any further information I can supply re power levels on the card to
try and see what causes the extra power / heat usage?