From my current comparison the Cobalt Flats are just not as lively, even when both are broken in. Lively, just not as. Like turning the lively factor fro an 8 for the Sunbeam (where a Hi Beam would be a 10) to a 5 or 6 if that makes sense. And, to my ear, the Sunbeams will maintain a certain 'life' or bounce for a long time, mellower than a Hi Beam, but that's by design. The Cobalt Flats are still an enigma to me. They are a very good sounding string. 'Meaty' and complex on my Lakland P style 5. They have a charachter unlike any other string I've played. But the differences are more subtle than the usual round vs. flat worlds. I am not 100% sure that I love them, but I do know that I do not hate them. They are fun and interesting though.
That said, the ever increasing stories we are seeing here of strings breaking at the peg on installation are disturbing. I had no such issues but enough folks are to be wary.