I often put stuff (read as math) in a custom UV just to have it computed faster (gpu vs cpu) even when it doesn’t really need it.
In a way, “doing stuff” to the UV and re-sampling is maybe even 1/4 the cost of an extra texture… milage may vary.
Gfx cards are way faster at computation than you’d think. In spite of the 40x series bringing nearly nothing if not a higher cost to the table 
Either way, as an extra tip for Op, just set your editor to Shader Complexity mode, and fine tune the material.
As a general rule of thumb for large (landscape components, around 100m^2) you get in the red with 9 textures (not necessarily just samples).
For landscape particularly, more layers = more complexity = shared wrap costs you more just because you can apply many layers to a single component…