We kill our Hebs (entire males) at 16 months with no problems of taint, and have done a couple of Soays too. If they go in August then the breeding season hasn't started so they don't smell - some breeds seem to smell more tuppy than others and I don't know if this is reflected in a taint to the meat.
I agree that it can be impossible to ring a Soay tup within the first 7 days. Even if you do get the ring on, the testicles themselves are so tiny that there is the risk they will slip halfway through the ring, which would be agony, or they can slip right through and you have a rigg, with functioning testicles inside the abdomen. I would argue that for welfare reasons, Soay should be ringed at three weeks.