That is an interesting question to be sure. On a Thermal clip on, your looking at a screen with your day optic and not through an I2 tube with a Night Vision Clip on. Now some will argue your looking at the phosphor screen in the I2 device. But with the I2 set up you do look "through it" where as with thermal, your straight up looing at a display screen.
In this case the answer is... Don't know till ya try it.
With Night vision Clip ons there are two schools of thought.
1. You don't need the rails front and back on the same plane angle wise. the argument is "the technology is so advanced and amazing it corrects for all optical misalignment sins!" In my opinion this is completely idiotic. All the Risely prisms do is remove the shift of the Micro Channel plate by optical correcting the light path as straight as they can... AKA collimation. Here is where is gets sticky as HELL. Most clip ons are only rated to be within 1 MOA of POA and POI... so in reality if it (the clip on's collimation) isn't DEAD nuts 1000% accurate to ZERO shift, your going to be off (WHO KNOWS which direction and how much) and having the Clip on, off axis to the day optic is only going to magnify this. Sure there are anecdotal stories out there that it doesnt mater. Having been through the process to desgin and build a clip on now, Trust me it does matter.
2. You do need the rails to line up (angle wise) AND my little addition to this is you need your optics at the same FUCKING HEIGHT ! ! ! Its ludicrus to spend $8000 and up on a clipon and then not line everything up. Seriously its like buying a Ferrari and then running non ZR (99Y) rated tires. WTF OVER ?!?!?!?! The argument that "you don't want to change your rings" is (no pun intended) SHORT SIGHTED! The BEST resolution in an I2 tube is right through the middle. So... why look through the edge of the tube? where resolution is less and you have the added icing on the cake of optical light loss! Let me say it again, looking through the edge and not the center is shorting yourself on a critical capability. In fact THE capability most important in a clip on.
Did I mention?
RESOLUTION IS BEST RIGHT THROUGH THE MIDDLE!!!
So now we go to thermal clip-ons. The big huge flaming question here is this. How are they aligning and collimating anything? Risley prisms? or just moving the screen around? So literally this is going to be a get them and try it experience. I will shortly and report back!
Huge food for thought.