Maybe I'm late to the party, but this doesn't seem to work for me no matter how
I try.
My video card is Radeon 7770, kernel is 4.2.0 (it didn't work with any older
ones either), Ubuntu 15.10 running open source driver stack, 22" monitor
1680x1050 native resolution which is 16:10 I believe.

Even if set "scaling mode" to "full aspect" (or "center" for that matter) and
then switch to a non-native resolution the image is still stretched to full
screen without respect to the aspect ratio.
I've tried to change the connection (I'm using a DVI -> HDMI cable, tried a
straight DVI -> DVI one), the monitor, and even the distro (tested with the
latest Fedora Live), to no avail. It seems that the setting is simply ignored.

Can anybody kindly suggest how I should go about it eg debug it somehow?
Overall I have a gut feeling there's something that needs to be tweaked besides
the xrandr setting, but have no idea what.

Here's what seems to be an identical complaint:
https://bbs.archlinux.org/viewtopic.php?id=192423

Thanks in advance.