I noticed that, when i install the fglrx driver the screen font size changes like it would change from 96 dpi to 72 dpi. Now i didn't see any utility where that could be set. Moreover, with the fglrx (i installed from Debian repositories like in the catalyst instructions here is adviced), interferes in some way in the powermanagement (which in any case for the moment on this samsung is awful) taking me away even the possibility to use manually standby. *WITH* fglrx there is no chance to reawake the machine from standby: it activates wireless and then the process doesn't go any further.
Curiously, in a lubuntu 12.04 (kernel 3.2 based) it was exactly the other way around: With the catalyst driver i got back pm-standby and pm-hibernate (both only manual as well since the connection to close_lid in some way seems broken there too).
So i went back to the radeon driver with a Liquorix 3.6 amd64 kernel. It does not work as it should there as well, but at least i've one way to get the machine to sleep.
The machine is a Samsung 535 u3c with 2x amd64 apu
I try to stay away from the proprietary ATI drivers, because I never had much luck with them. The problem is to find the right combination between kernel, Xorg and catalyst driver. For Ubuntu 12.04 it seems as if they have found a working combination, but this is not true for 12.10, where people have huge problems. This is probably because the catalyst version doesn't work well with the version of the Xorg server.
In theory, the catalyst driver should provide better power management capabilities than the open source driver. But as you already found, this seems not always work out well.
I always prefer the open source drivers with Debian methods ..