From twitter the other day:
"Would still love to hear your thoughts on intentionally overreaching and where in a training plan it might offer the most benefit"
Thanks for the question. The whole of point of training in general is over-reaching, just how much we should over-reach is the art of training/coaching. This is incredibly individual, even in the timing, given career and family responsibilities, as well as climate and race schedule.
One athlete I coach is a school teacher who has his summer off. We use this period to focus on either Ironman Wisconsin, or Kona, as he alternates years usually with these events, qualifying for Kona one year, racing it the next. The fact he has so much free time, allows this to be an excellent time to over-reach more than we would during the school year. It also coincides with the 12-week lead-up into Ironman Wisconsin for him, so this becomes a perfect time-window for specific preparation for the race.
So to answer your question, the specific preparation period is most important, and if you can over-reach there, you'll see a better transfer of that effort into actual performance. But I also want to caution you to be careful in your over-reaching, as this 12 week period is CRITICAL, so a mistake in over-reaching can lead to injury, burnout, over-training, and missed training during this time.
If you've given yourself plenty of time to build slowly into this specific preparation period, you won't require over-reaching beyond a normal amount. Remember, the more time you have, the less risk you need to take in training. The less time you have, the more risk you must take to be prepared.
Consistency of training over the long haul will trump a few big weeks of training, always.