The scheduler isn't a traffic management scheme, it doesn't really have any intelligence to speak of.
I was, perhaps, overly kind when I described the scheduler as being WFQ. It doesn't have the smarts to 'selectively' slow heavy bandwidth flows, it's more that it shares bandwidth between flows and the way it empties buffers.
Essentially smaller packets, such as pings, get through more quickly than larger ones, such as a standard download.
On the upstream you can, certainly, prefer certain traffic, indeed it's needed as part of voice services, however it's of questionable value and is more inefficient in terms of usage of the resource to prefer pings or whatever over normal traffic.
This will be useful in DOCSIS 3.1 though and will I suspect present way more of a benefit than playing with upstream schedulers.