The point is that it's specially formulated to stick to PVC. I've tried painting plastics before and it may look good but it doesn't stand up to the finger nail test. We'll see if it's up to the temps under the hood. What are we talking about 200ºF? After all, it's not sprayed on the manifold. Shouldn't be a problem. Besides, you know how hot the surface temp gets under the summer sun.
BTW, if only the primer was "high temp", you're no better off. The other paint is facing the hot side of the intake.