I can chime in on this a bit. The highs of the UM2 are recessed but there is still plenty of detail in there. I found after a couple of weeks of listening, my ears got used to it and the UM2s sounded very natural to me.
I have on't briefly heard the gr07 and do not remember their sound too much to comment. However what I find important is that the um2 is better fit wise out of the two, the gr07 mk1 had a really stiff cable that got annoying. Although I know the um2s do not sound the 'best' they are for me the most comfortable IEMs I have used.