jaykay, our ex-lodger had done sponging and didn't rate it. Not a very good success rate and he was very uncomfortable with the likelihood of infection. He felt it maybe caused fertility issues downstream, too. However it is still used by quite a few commercial flocks hereabouts, so it must work for them.
Given that your females share a long fence with your tup, would you not expect the majority to be tupped within the first 14 days anyway? I suppose you really don't want even one geld while you're trying to increase numbers, though.