In your exemplar, can you really separate the structural component of grammar from semantics and pragmatics? I don't think so. Repartee has grammatical aspects, to be sure, and perhaps one could come up with some regularities and rules that describe the playful banter of the (I assume) spontaneous, unrehearsed dialogue.
From my perspective, however, it seems the element of playfulness, a semantic/pragmatic component, is the guiding principle. The interlocutors are inventing the grammar as they go along, much the same as a child might in overgeneralizing some "rule" s/he perceives is at work (e.g., "I goed to the store with mommy"), minus the humor (from the child's perspective, that is).
Playing with language could take the form of pig Latin, the "poetry" of rap, and various word games involving rhymes, anagrams, feats of memory, imitating Yoda ("Getting stronger you are, Luke"), or doing what "her" and "him" are doing. It's a kind of one-upmanship in which each interlocutor tries to outdo the other by taking the "rule" that the other person introduces and then building on it within a pretty circumscribed vocabulary (viz., good, average, better, not much, and enough). The person who had the last word clearly had the punchline, and it's mildly ironic because what preceded it was quasi-precise approximating.
After reading your exemplar, a picture of a continuum popped into my mind. It is labelled "average" in the middle where the number value of average is five on a scale of one to ten, with number one being way below average, and ten way above average. The punchline's "exactly" is maybe a couple notches to the right of "average," say, six point five.
Years ago my daughter was learning English. It was a hot summer's day, and the air conditioning was running. A few minutes after adjusting the thermostat on the AC, I may have asked her if she was cool enough, and she responded, "Cooler enough." Now there's a "rule" I could have run with in a sort of verbal riff, had she been able to catch on and play with me. Intellectually, we were not on the same level, obviously. By the way, her locution makes really good semantic--if not grammatical--sense. "[I'm] cooler enough" describes "exactly" her level of comfort: not too hot, not too cold, just right. My adjustment made her cooler enough. Any more coolth and she'd have been uncomfortably cold; any less coolth and she's have been uncomfortably warm.
Perhaps you could characterize the grammar of your exemplar as "adjectivality." Change the words to "brilliant" and "superior," and you could have the following:
He: "Should we keep Casey as our clean-up batter?"
She: "Well, he's had a brilliant season thus far, right?"
He: "Well, definitely more than superior, but certainly less than brilliant."
She: "I'd say a little more than more than superior, but a little shy of brilliant."
He: "OK, why don't we settle on a little shy of shy of brilliant, but highly superior."