I'm pretty certain the second part of my comment sums up my answer. Is it an upgrade if your connector wiggles loose during a show? That's a decision only you can answer
In any case, the mac's GPU's are a joke and are notorious for overheating, so yes, any old GPU you throw into an eGPU would probably be a major upgrade. To answer your own question, just compare whatever GPU you are thinking about using to your macs current gpu, I'm sure it'll be an upgrade, just don't wiggle your cable.
Also, OSX has dropped support for Nvidia (Which Resolume works best with), So yo'll probably want to do some research on the forums for what AMD cards work best with Resolume, Or email in, I'm sure the guy have AMD recommendations.