I'm not sure this will help, but. "He talks the talk" has this implied word: "He talks the (right) talk". Meaning he is at least saying that he knows what he's talking about, or that he can do the job. If someone "walks the walk", he is demonstrating that he can; making good on his claims, in other words: showing his capability.
I think positive or negative usage depends entirely on the situation. If, for ex., I gave out with all sorts of b.s. and then claimed that I was qualified to be the President of the United States, I imagine there would be many negatives uses of, "Yeah, but can she walk the walk?" People are expressing doubt about my capability. If I took charge of organizing a fund-raising dinner and it came off without a hitch, then people could say, "Yep, she can walk the walk, all right". Or possibly that I could walk the talk, which I agree must be a contraction.
