No, it isn't time to play Austin Davis. The team can keep moving forward with Clemens. They may not win much but all the guys that need to get more experience on offense will be able to continue with what they've been doing. With Davis, that wouldn't be the case.
And I'm not so sure it is "known" what Clemens' ceiling is. As it stands, he's a solid #2 QB in my book. He's got plenty of years left and for me, my plan is to have Bradford under center for the Rams for another 10 years or so which means Bradford/Clemens is fine for the next few years. Surround them with a good team and then when the team is consistently good, a developmental QB can be drafted. Then he can come in and actually develop. Throwing Davis or any other greenhorn out there with receivers who aren't ready and an offense that isn't particularly good at blocking isn't going to let anyone see what the QB can do.