I'm sure it'll have an impact, however I do not believe it will be significant, probably not noticeable at all I'm guessing. You just do not move that much water through the loop, when it kicks on it runs for maybe a few minutes at a time. I'm guessing it is similar to someone taking an extra shower each day. Plus it isn't cold water going back to the water heater, for mine it was usually still 90*+.
