What is the 3D printer filament (or pellet) most resistant to bending at high heats?
Your expected operating temperature exceeds the glass transition temperature by 3&nbsp;&deg;C. This implies that the structure will become weak and can deform under load. 

Note that you cannot simply print PEI on a normal machine, it requires a special high temperature capable printer with hot end temperatures up to 400&nbsp;&deg;C and heated bed over 120&nbsp;&deg;C up to 160&nbsp;&deg;C, furthermore it will need a heated chamber (up to about 80&nbsp;&deg;C) which requires special care to cooling and placement of electronics and motors.

Not having specified what kind of structure you require, you could look into steel.