Based on your description it seems to work as it is supposed to. Sounds like you are adjusting the rendered output, not the actual texture, which are two completely different things.
Are you applying this adjustment above a rendered 3D layer or against the un-rendered preview?
If you want to apply a curve to the actual texture - then Edit the texture. In the properties for that material click on the small folder next to Diffuse and then Edit Texture. It will open as a .psb file with a UV overlay showing how the model is unfolded on that texture. You can apply layers, adjustments and smart objects in it as you can any other .psb. To preview the effect just save it and switch back to the 3D window.
Thanks for the quick response! Here's where my confusion comes in...
I can make a gradient on a layer and merge it down onto the model...
But if I try to get the same effect with a masked curves adjustment layer, the effect doesn't drop when merged...
I'd like to do this in 3d and not 2d space as the model I'm working on has multiple textures mapped to it and I want to apply the effect seamlessly across border edges.
If this is possible, I'd love to know how to do it!