After reading this thread, I have finally found some time to download and play with RawTherapee. Really liking it!

Haven't tried six stops, but did do five with 5D2 and 5D3 as that was the maximum LR4 would allow. Interesting results for 5D2. Not pretty, but some of that was from the processing method in all fairness. Some banding showed up, as did a lot of colour splotching. The histogram changed from smoothish curves to being heavily notched. It appears that it applies a multiplication factor with no attempt to smooth the transitions. After seeing 5D2 results, I thought it might be best to try another piece of software rather than convert 5D3 shots to DNG and then push it up. Should mention that the high ISO shot also had less noise in it.

Tried Adobe Camera Raw current release candidate next as it 'supports' both bodies. After two stops of pushing 5D3 shots I just gave up on it. Plasticy look of over processing killed my spirits. White balance was affected in a notably cruel and unusual way as well. Did not use for 5D2 shots due to disgust.

DPP only supports two stops worth of adjustment so it wasn't a possibility.

RawTherapee resolves the colour blotching, but not the banding and noise that appears in the 5D2 shots. 5D3 is not yet supported. Previews are generated, I assume from the JPEG thumbnail in the RAW file, but when I opened the raw file it had a very unusual pink cast to it. Was able to do some cleanup on the 5D2 results, but not enough to think I'd ever want to exhibit the results. The banding and noise graininess is quite noticeable at 50% view.