The developer updated the game. It runs now really smooth in every level with the open-source driver, but with Catalyst 13.4/13.6 beta, I can't even start the game. It segfaults.
Also I forgot to mention my video card, which is a Radeon HD 6670.
For me it works with Catalyst OpenGL 4.3 beta and with Catalyst 13.8. I'm using a Radeon HD 7870. Maybe you could give it a try with these newer betas and see if it works now.