Well then... what's the current up/down internet connection, after subtracting the average use? That's the limit (unless limited further by something else). Why not advertise it? It's probably huge.
>... what any single machine can push out ...
That number wouldn't be very misleading, and could actually be useful - it's effectively the limit on a dumb fileserver. If their code results in a lower boundary, that's their fault, not yours, and not in the least incorrect because it's being restricted by them.