The qualities that allow a student to get a 2400 on the SAT aren't necessarily ones that will contribute that much to overall college apps - sure, it means they're "smart," but there are plenty of brilliant kids without 2400's.
the average student who scores a 2400 is significantly more accomplished/intelligent than the average student who scores a 2390.
A somewhat indirect way and theoretically inferior although certainly much more practicable way of testing the hypothesis would be to survey America's more accomplished high school students possibly over several years to get a decent sample size (RSI, Intel, IMO, maybe some others) and determine the ratio of 2400s to 2390s among this group. If the ratio was significantly greater than the overall ratio (about 1.9 for the class of 2011) it would lend support to the hypothesis.
Getting an 800 on the SAT is a random event – one needs to make zero mistakes in the course of answering 54 questions at a rate of 78 seconds per question. Indeed, the College Board reports that only 15% of students who retook the math SAT after scoring 800 scored 800 on the retake. The average retake score was 752 (which is only 11 points higher than the average retake score of students who scored 760 on their previous attempt).
I take it you deny the existence of people who can easily get 2400s without studying much or at all. If the hypothesis is false then the difference between 2400s and 2390s would be just a small amount of preparation and luck as you said.
n general I think a disproportionate number of the super-high SAT scores on today's exam (and to a lesser but significant extent on the older tests) are coming from perfectionist categories such as high school valedictorian, college admission obsessives, masters of test prep, immigrant cram school attendees, and doctor/lawyer wanna-bes. The grade and test equivalent of compulsive hoarding is rational to the extent that some scholarships and admissions are directly based on it, and there is less of the low hanging fruit to grabbed at university level and beyond.