Here we go again another jerk wanting to throw out the baby with the bathwater. "When the 300G has matured." Did they finish the 300 standard edition yet? Is it working at the same speed or better with respect to the Window's XP driver?
You have very harsh requirements. An open source driver will very likely never match the proprietary blob in terms of speed, simply due to the lack of manpower. Expecting it to be better is extremely optimistic.
But r300g is already a better driver than r300c. And it is pretty feature complete.