Do National AP Scholar awards (4 or 5 on 8 AP exams or more) look “really impressive” for college admissions, or do they not really matter/provide no real edge against other candidates who don’t have it?
I would say no to “really impressive”. I believe there are thousands of students who fall into that category every year. Some high schools also limit the number of AP courses younger students can take, and colleges wont hold it against applicants.
I’ve also said on other threads, applicants need to eliminate the word “impressive” or similar words from their vocabulary until after decisions are made. It is the rare application/course schedule/EC that will “impress” admissions. Perhaps the AO that read Malala’s application was “impressed” but she set the bar high. Do things that are right for you.
It will provide no edge. It will make you and your parents feel good, and the corresponding college credit is great, but it’s really just another piece of paper. Maybe a bit more prestigious than a Perfect Attendance medal. 
Just think of it this way - probably 80% of the applicants at a highly selective school will have an AP scholar (distinction/honor/etc) award. How many will have a National AP Award - maybe 5%. Seeing it on one of twenty apps may not make it unique, but it is certainly better than the typical AP scholar.
Same with NMSF - it’s slightly more highly regarded than a Commended National Merit Award.
Are any of these the golden ticket into HYPSM? Absolutely not. Are they ‘really impressive’? Maybe just mildly impressive. In the end, these are all just pieces of the holistic puzzle.
What about a student that gets the 1st “5” AP score in the school’s history for a subject?
"'What about a student that gets the 1st “5” AP score in the school’s history for a subject? "
Considering that AP scores of 5 are not unusual for thousands of college applicants every year, I’d say it will be viewed along the lines of someone winning the first “Harvard book” award for a HS.
Perhaps impressive for that school- not impressive as far as overall college application are concerned.
Its a good addition to your application if you have all 5’s and have National AP Scholar award by junior year. Most get it in senior year so its of no value in admisdions process. If you pair it with NMSF, high GPA, high SAT, high SAT II then you are a pretty rare breed, it shows you are among <1% of distinguished students. You can get into really good colleges and get good merit scholarships.
However, HYPSM’s holistic admission squad doesn’t care as much, they rather go with someone who can throw ball, has right complexion or grew up in right geogrhapical area. It matters more if your parents went to their school or didn’t go to college at all, ivies arent too interested in mediocre parents, just both extremes.