mesa compiled with "--enable-texture-float" option is basically needed to be able to "run" the unigine engine. it just does not make fun when the fps are below 10 fps. if you want to play a slideshow feel free to do so. as i worte scripts to test oss drivers i know how long it takes to compile those, also i have got scripts to install and remove binary drivers. with debian i just completely uninstall fglrx when i want to use oss again, but that does not take longer than 30s + reboot. install takes a tiny bit longer, but as it is fully automated it does not matter much. with u you would not neven need to uninstall fglrx, you just use "update-alternatives --config gl_conf;update-initramfs -utk all;ldconfig;reboot". same way back, maybe a xorg.conf is needed for fglrx. if somebody is unable to switch the driver when it is needed i can not help, fglrx is maybe not the best driver, but when you really want to play a game with unigine engine then fglrx is the way to go.
I feared it was still too slow
Unfortunately fglrx is not an option because it does not support latest kernel and driver stack, so I just cannot use both drivers.
kernel support is basically possible with patches, it crashed however a few times with a daily kernel, then i removed the card with .39 it was stable. depending on your distro it is easy or a bit harder to get the patches.