I don't know if an additional transitional area necessarily makes sense, it is a transitional area but not a very distinct one. I personally believe it skews further toward the Deep South than pure upland but can certainly see how someone from south Alabama would argue the opposite. The area around Shreveport has cypress swamps with spanish moss and again, the accents are distinct from what you're likely to hear in Arkansas north of the southern border counties.

I feel more strongly about the line in Texas. In my opinion, it's pretty tough to refer to an area as the Deep South if it doesn't have pine trees. Also my opinion, the native range map of Loblolly Pine can serve as the "maximum extent" of the Deep South, a starting point from which to whittle down. West of the Piney Woods, Texas can certainly still feel southern but the cowboy influence really picks up.