gene cooley
Autocrosser
O.k., then I think you might see a significant difference in temps between that point (air filter lid) and the intake manifold in general, with the manifold being hotter. I don't know if between the throttle body and the runners there is as much difference, for a cross flow head (I think that's what you have). However with a counterflow head (X1/9) there will be a big difference in temp due to the runners being right over the exhaust manifold, as Huss siad. To me that is all the more reason to add the insulator spacers at the head rather than between the runners and plenum; try and reduce the runners' temp as much as possible by insulating them from the head AND by adding a heat shield between it and the exhaust. But as you noted, your plastic intake manifold may be a much better insulator and therefore the runners could be much hotter than the throttle body.
Maybe this will clarify things:
Added a pic of the intake manifold. It's one piece that includes the throttle body mount, a plenum about 8" below the tb, and the runners that wrap around the plenum and bolt directly to the head. All one piece.
Ignore the 72mm tb, that has been replaced with a 58mm.