Anything outside the mainland restarts the clock, including HK or Macau. All they want to see is that you got checked out before the deadline. I was in on a tourist visa in 2009 and had to do this. If my memory hasn't failed me, I vaguely recall it was a 30 day limit back in those ancient days.
If you go to Lowu/Lohu, you'll have to take the East Rail line down one stop before coming back. Some of the other HK border crossings will let you do a u-turn just after clearing HK immigration and customs.
Or, since you went to all the effort to cross the border, spend a day or two playing tourist.
