On the LFX engine there is a composite intake manifold, why would you fit insulators ?

While the insulator does assist with blocking heat transfer due to conduction, it’s nowhere near as effective on the LF1/LFW/LFX engine due to their plastic composite intake manifolds as you’ve alluded to. However the primary reason for these insulators achieving gains as described, is not to do their ability to prevent heat transfer from occurring (though this does mainly help with power consistency along with modest performance gains), but it has to do with the fact that we’re lengthening the intake runners. This ultimately improves intake manifold efficiency in the usable engine operating range, as the design of the intake manifold from factory is better suited to engine speeds beyond that of the factory rev limit.