Yeah, the RAE really liked Research Groups because it showed that you had a plan. I'm sure that the REF is going to look at them in the same way.
I'm not so sure for two reasons. The first is that it's not clear that the REF will have the equivalent of the RA5a which was the awful document in which we had to describe organization, plans, support, etc... Indications from HEFCE is that they will be providing templates in order to "reduce burden" and hence the nature of the templates would strongly influence the answer to Qrypt's question.
Second, the proposed subpanels in the REF are very large and include quite disparate groups - one contains both subjects allied to medicine and nursing, for example. With such a structure, it's hard to see how synergies can be demonstrated and hence I think that smaller scale coherence may also become unimportant.