I have the Jetstream, and it would serve me well in south texas for sure, layered appropriately for the weather. I would be comfortable in the Jetstream by itself down to the low 50's, with a thermal shirt probably into the low 40's and with a thermal shirt and another layer into the 30's...sitting in a blind can make that even more comfortable than sitting in a tree stand/tripod.
Given the way temps rise so quickly in the south, I would be inclined to buy less insulation and more layers, that way you can de-layer as the day goes on and keep the outer layer on. And if you plan to do any moving around (I know, South texas doesn't lend itself to that kind of hunting, but maybe...)then less insulation will be your friend.
When I hunted down there near freer, it wasn't uncommon to have temps in the mid-high 30's in the AM reach low 70's by afternoon...it isn't like the panhandle where it goes from teens to 30's.