Trachhead. 20% of all HS guys? I'd say that's way high.
Have you ever seen those Lowell vs. Washington meets?
The SF section may have 5 guys under 5 in a good year. Being generous, you could add another 10 who don't do track but play soccer or other sports.
Then you have the private schools. SI is probably good for 20 guys under 5 and maybe another 20 in all of the other private schools. So round up to about 50 HS guys in SF.
Now take the college teams (SFSU, USF, CCSF), and you might get another 15 sub 5's.
Club teams with runners in SF such as Excelsior, WVTC, and a few random Aggies & Transports & unattached guys like Balkman & Lundstrom will give you at most 25 sub 5 runners.
What's left, the triathletes and unknown guys. Former college runners who don't race anymore, but still jog a few days a week. Not too many of these guys could break 5. I'd say at most 20.
So, my estimate for SF would be 100 sub 5 milers.
But that's not representative of the US as a whole. Take extremes such as a college town with a good XC program and a lot of running alumni still around and you might have 100 guys under 5 in a city of 25,000. There are also probably cities of 25,000 who may only have 1 or 2 guys who can break 5.
Really tough to guess.