Funny you should ask…
Heh. This was the topic of an abandoned thesis in grad school. I thought i could make the case for sophism, but as I accumulated evidence, it became exceedingly apparent that the reverse case was stronger.
I am familiar with the works you cite. Part of the attraction of "The Case for Sophism" was that body of scholarship was sufficiently small to be encompassed in a few months time, and the dialog had been ongoing.
We mainly hear of sophism from its detractors. There is precious little work by the sophists themselves; and, while their work does paint a different picture, its just not enough to justify what was merely a hunch on my part.
However, all that research did pay off in the form of another cosmic realization; that is, Athenian democracy, indeed, Greek democracy was deeply rooted in an intensely agrarian tradition, and well, if you know farmers, they are generally suspicious of flowery language and crafty arguments.
Irony of ironies, Socrates was routinely portrayed—via thin disguises—in the works of Aristophanes as a sophist! (Socrates attributed the beginning of his troubles to Aristophanes' plays.) In another example of the rhetorical power and representation of popular perception via theatre, Sophocles used the character of Oedipus to portray Pericles. Again, the kind of guy who could not leave well enough alone, who HAD to KNOW! And, who was famous for entertaining intelligencia from all around the world. He in particular was blamed for leading the Athenians into the disastrous Peloponnesian War through his fantastic powers of persuasion.
Having grown up in a farm town in the Central California valley, I was well acquainted with the farmer's perspective. Like the Spartan he admired, "Deeds, not words" were the measure of a man. Their pride, power, and prejudices permeate Greek literature. Americans, in general, will be familiar with a similar bias, I think most would agree. Also, that's not necessarily a bad thing. Farmers build great public spaces, and they are generally trustworthy, capable neighbors.
But the topic of sophism is ever renascent. Robert Pirsig's Zen and the Art of Motorcycle Maintenance brought the topic back to the public eyes, yet, as the protagonist himself admits, he's a poor scholar. The words were not backed by deeds.
At least any that I could find to a sufficient degree to "win" the case.
And so, we are left with an opinion. Maybe they weren't so bad. Maybe they were onto a more scientific (as in bottom up, starting with observations rather than top down, beginning with postulates) form of inquiry.
Unless some new text emerges from the sands of Egypt, it is not likely we can resolve this, the pejorative meaning of "sophist" in modern usage is like to stand.
[What's wrong with this little essay: Sorry, I'm not citing things in a scholarly way at all. Any criticism of that is justified. I am away from my notes, and typing on an iPhone is tedious, so, excuse the incompleteness of this response, if you will.]