I keep waking the horse up to beat it some more.
There is design, and there's execution. If you haven't read or gathered that these bikes are assembled somewhat more casually than those assembled in say Austria, Munich, or Hamamatsu, you're not operating with the same context as the rest of us. If you've not read of engine and transmission issues on other 390 forums, then you should. I suspect that when you refer to the 20 mile and 1200 mile guys, you're not specifically referring to RC390 owners.
Your assumption of "cleaned and run" is not one that is necessarily correct, at least not to the extent you're willing to believe. Were that true, I see no way that these engines would produce the amount of crap at the first (sub 300 mile) oil change. I say that with absolute conviction, based on breaking in Japanese, German, Italian, and British motorcycle engines from brand new. I have never seen that much crap in the oil. It might take a large defect or mistake to break an engine, but it takes a lot less to shorten it's life or reduce it's performance. You can't honestly think that having metal shavings, flakes, gasket material and other spooge circulating in a new engine is somehow good for it, OR part of the manufacturer's plan.
As to 600 miles being a "calculated" or "best" anything, I suspect that's one of those "it's the way we've always done it" numbers. Contrast that to today's Porsches. First service: 10K miles. The precision of manufacturing (in the Porsche) is an order of magnitude greater than these engines. That is a calculated interval.
Obviously YMMV. I have no expert training, but I do have a lot of motorcycle experience, and I'm at least as well read on the subject as the next guy. I am of the considered opinion that my engine will run and last better having changed the oil earlier rather than later.
Since beating dead horses is so much fun...
Both the 20 and 1200 mile bikes are RC 390s.
Would differences be found if the engines were torn down and examined in greater detail... perhaps, but until a report of a ruined engine appears its probably not worth the trouble.
The screens and magnet are great at collecting junk and perhaps if more bikes had them more material would be noticed on other bikes as well.
The amount of material on the screens must be acceptable to engineering as a cost trade off in the manufacturing process, otherwise one would think the engines wouldn't be released to the public.