Sometimes things may appear to work when actually they don't. Many years ago, I was working for Georgia Tech and wrote the book, Radiant Cooling to the Night Sky. It didn't make the NYT Best Sellers list. Maybe we can dispel some misconceptions without a lot of fussy math.
Reflective surfaces like Reflectix only work with an air space. It can be a very tiny air space but if the reflective surface touches, all heat transfer is by conduction and the reflective surface has no benefit.
The R-value of Reflectix is about 1.0. That's roughly equal to the same thickness of cardboard. This amount of insulation is only delaying the inevitable temp equalization for a few minutes.
Radiant heat transfer is only a function of surface material. If you paint Reflectix, the emissivity is now that of whatever you painted it with.
What we actually want is stable OTA temp. It doesn't much matter what that temp is but it's almost impossible and certainly very impractical to achieve when using the OTA. So, we settle for ambient temp with the assumption that it changes so slowly that we won't create tube currents and can call it stable even though it changes somewhat through the night.
The faster you cool the OTA to ambient, the sooner you eliminate tube currents. The OTA will reach ambient and insulation only delays it a few minutes. Insulating the tube actually creates more currents but we're getting pretty picky at that point.
The big issue is radiant cooling to the night sky from exposed elements of the optical train which are aimed at deep space creating a huge delta T and the situation that we are generally observing under clear skies which reduces atmospheric attenuation and gives us pretty much maximum radiant transfer from the OTA.
We are dealing with very small amounts of energy here. The transfer doesn't correlate well with anything except sky temp. When you simplify a lot of data and math, the cooling can be expressed as (-0.59 * Sky temp) + 44.21.
In English, about 11 Btu/hr-sf which is about 3 watts. That's about a dozen wooden kitchen matches per hour. It doesn't take much of a dew heater to overcome that.
If the case of a refractor or SCT, without a dew shield, the front optic is exposed to a large part of the sky dome. With a dew shield it's seeing only a small fraction and heat loss is greatly reduced. It's not about insulation of the shield, it's just about blocking as much of the sky dome as practical.
If we get into really tiny numbers, the dew heater does create a warmer boundary layer of air at the front optic (on both sides). A dew shield with insulation does protect that boundary layer but the first little breeze pretty much wipes that out regardless of how much it insulates.
If you split hairs, the bottom of the OTA is receiving heat from the ground while the top of the OTA is losing heat to the sky which creates a very tiny difference in tube temp and a very tiny convective current. Insulation won't stop that either.
There is no reason for Reflictix but if it works you, by all means use it.
It would be interesting to to take a useless SCT and put thermocouples in the tube and on the optics and actually measure what's going on with different configurations, angles, and sky temps. Until then, the numbers are just too small and the variables too great for a conclusion.