GaaraOrDSand; Good day to you.

Be aware I am far from an expert on graphics drivers. however, I am aware of a few things.
This is my thoughts -> that card should scream. To that end ->
1. What results from:
sudo aticonfig --initial
As you are running a proprietary driver.
2. What does AMD/ATI web site advise as to the proper driver to use in your instance:
If you go to ATI:
And use "autodetect" when you are looking for a Linux Driver, what does it say about your chipset and any suggested driver?
Bear in mind --departing from ubuntu's repository is in my mind a means of last resort. Doing so places the maintenance of your system as your complete responsibility, being aware of what updates break the system and how to repair it.

I remain open to discussion.

best regards