Well to be truthful once you have a proven 3D design, you can make it out of whatever you desire. Most of the time a 3D printed low temp thermal plastic is made for the loss casting to get the mold. Not to get wrapped up in the how you got there...
I am aware of several thermal plastics that can perform under pressure if the intent is to add boost. I agree metal designs will hold up better.
Also be aware that 3D printing isn't limited to thermal plastics.
A fairly new filament for FFF style printing:
https://essentium3d.com/products/metalfilament
There are caveouts to its usage, but its a metal finished product.
If you wanted metal 3D Printed there are services that can do that too until its a viable desktop solution comes out of prototyping stages.