I can't remember how much D3 the body can produce on it's own in one day if out in the sun. Something like 16,000 IU, all by itself. So no, it's not quackery. It's healthy to have your D3 at top of range. Even if you take 12,000 IU's per day, you are not overdosing.
People who are taking ultra high doses of D3, they're only doing it for a short term until they reach a maintenance dose. And yes everyone is individual, but you will not OD on 10-12K IU's per day, if the body is capable of making that all on it's own in an optimal setting.