Well, I don't have experience with PEX delivered as tubing, but I worked as an engineer for a flexible pipe company that used PEX all the time for the pressure containment layer, so I know the basic material well.
PEX maintains full properties up to about 180 deg F; higher than that, it starts to lose its strength. As far as I know, it can only be rated at 80 psi @ 200 deg F if it is also good for at least 120 psi at 180 (just make it thicker, and it can handle a higher pressure/temperature combination, and as long as it's set in concrete or something such as in a hydronic floor, it's really no problem).
However, 250 F is well outside of its capabilities at any pressure. It will deform plastically (no pun intended) at that temperature with basically its own weight, and if I remember correctly, actually starts melting at 255-260.
The only plastic we used that could withstand 250 F was called COFLON (not sure of the generic name, but it was a variant of PVC-C), and it was around 7 times more expensive than PEX per kilo of raw material, so the copper might start looking better and better.
Best Regards,
Dave