I wish I had an answer for you on underclocking. Have you tried running the faster one off of BFGminer yet? You mentioned a speedup with CGminer on your 550, I'm curious if it will run slower and cooler and if there is as big a difference in speed on the 700.
Very odd. First time ever that on default clocks that if my software runs faster people are looking for ways to slow it down. Let it be known that BFL never informed us of any messaging protocol to over or underclock the devices, so I can't see how we could even implement such a feature. If it runs slower at the same clocks that implies a software limitation.
You would have to ask Nasser about that, I know he talked about it.