Hey Peter,
Yes indeed, Tim's stuff is spectacular. I too have one of his compressors, and I would certainly be using it in Mastering if either of two criteria were able to be met:
1) I would get a call for mono mastering
2) I would get a second 670 to match.
As for the Manley, it does a good job, but I will replace, or more likely, augment the MP with a new Lucas Mastering Series Programme Equaliser that we're working on.
In a perfect world, then I would add Tim's 825 Stereo Mastering Equaliser.
Still searching for the "perfect" analogue valve Mastering compressor, and hoping we don't have to build that as well.