I agree that for high school boys, sub-17 is much more difficult than sub-5.
More purely anecdotal evidence...
My high school team mates' personal records, achieved over a 3-year period or so (not saying we were fantastic, just that we were in line with what everyone else is saying)...
4:40 - 16:5x
4:41 - 17:01
4:42 - 17:30s
4:43 - 17:09 once and only once
4:48 - never broke 18:40
Our most frequent cross-country course, that all of us knew very well, was a mix of grassy fields and dirt trails with almost no elevation change whatsoever. People always complained about the "hill" at mile two, but 10-15 feet of elevation gain over 200 meters isn't really much of a hill.
After we graduated a stronger crop of runners came along. Several ran in the 4:20s and 4:30s. But only four of them ever broke 17. And a 4:35 runner who never broke 17 in high school has since run about 15-flat on the track.
I agree with the post that there is a difference between cross-country after inconsistent summer training and spring track with consistent conditions and months of build-up, but we usually had pretty good weather for most of our cross-country seasons (minimal rain near race days, often in the low-high 60s on race days) whereas our track season was one rain-soaked meet after another, cold and damp, with no good racing weather until late May or early June (when any of our seasons ever lasted that long). So if you want to consider summer training versus year-long progression, I think you may also have to consider other conditions like weather.