Anybody have a guess on what is the rating difference here ??

They are basically close to the same weight. Raw materials 5 oz for XP per SY vs 4.8 for IX. Three layers of IX is actually more expensive.

I have one piece of IX so I was also thinking of a side zip open-able UQ with tie in liners.

Made my top quilt similar. Sort of a modular setup.

That way I could add 1 layers of IX to the Climashield UQ. What would the rating of that be ??

How a guess with 5XP + 1 layer of IX next to the body + 1/4" GG foam pad in a double layer ??