Herbst notes that some Fermi GPUs have engine re-clocking working, but it's memory re-clocking where things are frequently breaking. While ultimately the memory re-clocking will need to be figured out if there's to be good performance for these aging NVIDIA GPUs, as an interim step Karol is proposing that the problematic Fermi GPUs should at least allow partial re-clocking. This would allow the engine clock speed to be ramped up while the video memory speeds would still be low, rather than trying to force them up too, only to run into problems.
However, without re-clocking of both the GPU core and video memory, the performance will still be less than ideal and far behind what's provided by the proprietary NVIDIA driver. Karol's testing with a GeForce 630M Fermi card he found the frame-time improved for GpuTest's Pixmark Piano and that the Unigine Heaven frame-rate rose from 5.1 FPS to 6.4 FPS.
The patches for supporting this partial re-clocking can be found here, but it's not clear based upon initial comments whether it would be accepted upstream since this is just half of the way to re-clocking rather than just focusing on figuring out Nouveau's struggles with Fermi memory re-clocking as the proper solution.