Brakes may be a pretty straightforward thing to control, but the amount of cable pulled for a given amount of lever movement is an important variable that affects their feel a great deal. This changed significantly between 6700 and 6600.
Using 6700 levers with 6600 brakes will make it feel very different. It will feel much more soft/spongy, but you will have sufficient power if you squeeze enough. The opposite combination is not recommended - 6700 brakes with 6600 levers - you then have a hard/solid feel, but very little power.
