Dell Vostro 15 7580 144 hz via thunderbolt possible ?

There are some systems that have at least one display output port controlled by the NVIDIA GPU specifically to support things like VR and G-Sync.  However, on every Dell/Alienware system I've seen or read about thus far, the USB-C/TB3 video output is always controlled by the Intel GPU.  The only exception is the more recent Precision 7000 Series models that offer a BIOS option to allow you to choose which GPU controls the display outputs.  If you want to be sure, try connecting a display to the USB-C/TB3 port, open NVIDIA Control Panel, and go to the PhysX Configuration section.  That area has a diagram showing which GPU has control of which display outputs.

If the Intel GPU controls the USB-C display output, you would not be able to use G-Sync or even Adaptive V-Sync.  And in fact I'm not even sure you can use 144 Hz.  The highest I've seen confirmed is 120 Hz on Intel GPUs.  But even if you could, running high refresh rates without G-Sync or Adaptive V-Sync can become a bit of an issue, because your only options at that point are V-Sync On or V-Sync Off. The former is difficult if you’re running a high refresh rate unless you want to reduce detail settings to make sure the GPU can always sustain that refresh rate (otherwise you will see judder) and the latter is difficult because it allows frame tearing. This is the exact dilemma that G-Sync is meant to solve by allowing the best of both worlds where you have no tearing AND no judder because the display’s refresh rate will adjust based on what the GPU can deliver, but that wouldn't be possible when the Intel GPU has control of the display output.