Try this. What ruins people at Boston is the 1-2 punch of the downhills out of the gate and then the other downhills at ~21 miles (and you can add the descent at 25K if you like). The effect of the hills is like radiation poisoning; once you've overdone it by opening up your stride too much in the first 20-30 minutes of the race (depending on your pace), the effects of the pounding, while not yet evident, are irreversible and WILL show up later.
Now, elite runners can almost outrun this effect, because even if they shit the bed they're done in well under 2 1/2 hours. Not so for the sloggers. When sloggers get the boost from the tailwind, it pushes them down the hill just as it does faster runners, but they still aren't fast enough to outrun the mechanical damage thus incurred. Someone who in theory might go from 3:30 to 3:20+ thanks to wind alone is going to still get badly beat up starting at about two hours in, right at that shitty downhill leading into Newton Lower Falls. Sure, they haven't burned any more glycogen in taking advantage of the wind early on, or gotten more "tired," but being shoved along still puts a big strain on the chassis even if the engine's not taxed.
So this, in my view, is why the sloggers don't get quite the same boost from the wind *at Boston* as the elites do.
In terms of downhill races and sloggers vs. elites in general, the sloggers actually gain *more* of an advantage than they elites do as long as the race is short enough to render mechanical pounding a non-issue. Look at the Millennium Mile up in New England every year. The fastest guys are usually 4:00-4:10 types on the track who pick up perhaps 10-12 seconds from the hill (not counting wind, which can obviously go either way). If the 6:00 milers got a similar percentile boost, they'd gain about 15-20 seconds. But it's clearly more like 30, and the peoplle who can't break 7:30 on a track are almost a minute faster. The explanation? Fast runners are already mechanically very efficient and moving close to as fast as they would if not limited by fitness, whereas the sloggers might be just as "fit" by cardiopulmonary metrics, but don't have the drivetrain to get it done. So they can fall down the hill to greater relative effect than can runners who are already smooth machines.