I have a Thinkpad T400 w/HD3470 graphics adapter and 1440x900 resolution sitting in a port replicator (DVI out available, external TFT with 1280x1024 resolution). The laptop runs Ubuntu 9.04. First I tried the radeon driver which gave an average 3D performance (not too important) but very high temperatures, even while idling. Yes, I tried the new DynamicPM and ForceLowPowerMode - the temperatur goes down but is still around 55degC. On the other hand switching between single head and big desktop via xrandr works like a charm. Even after suspend/resume w/o having to restart the X-server.
With the catalyst driver (v9.6, 8.620) it's the other way round: very good performance, very low temperature (40-45degC - at least 10degC cooler!!) but much trouble whith the secondary monitor. X only starts up correctly if the external TFT is disconnected. Then I can switch the secondary display on via xrandr and use a big desktop. But after suspend/resume I have to restart the X-server - otherwise the external TFT would stay black, even though it is listed by xrandr. Even disabling and enabling it via the gnome multi-monitor utility does not help.