I would argue that communities with quality posters and healthy moderating accomplish much of the desired functionality for willful subscription to topic-centric posts, but I see where you're coming from.
I'm not sure I see how a topic channel would change this, though, as I see a variety of say, civil engineers
posting under a related topic, and a person again being lost in the river of content.
To me, locating content from a specific person comes back to searching. If I search for "David Metcalfe Google+"
in the top search bar, I can quickly locate every post I've put out there on the topic, and the amount of content varies naturally based on what circles have what content, and the searcher's presence within those circles. Similarly, if I'm in an Engineering community, and I want to see everything from you on civil engineering
, I can search for that in a community as "Rob Ferguson civil California"
or other combinations of terms.
Further, one can save searches like these that are outside of communities, and quickly pull them up later as desired. I'm sure this will come to communities as well, given how large and unruly some of the content streams have become.
Between the two, I'd rather work with searches, and see Google include more powerful searching abilities. To pull off topic channels in a more precise way for yourself, I'd probably lean towards the use of hashtags, since they're supported and undervalued on G+ so far.
If you still feel the above is insufficient, perhaps you could elaborate on the perceived differences for additional clarity so that I understand better.