In the case of LLG, I’m quite certain the charts on their saver top quilts are the same as their non-saver Argon shelled models, just lacking the 900 fill numbers. My guess is that the HG Econ numbers are more accurate. My HG 20F underquilt came in as speced. My EE 40F topquilt also came in as speced.
For reference, my LLG saver 20 F 75″ L X 55″ W topquilt is 28 oz with the following weight increasing modifications. I added the dee ring pad attachments, the insulated hot footbox, the neck draft and 1 oz of down. The “same” size HG is speced at 25.3 oz. But I would not have been able to add the neck baffle, which I wanted. When considering weights, I have also read that going from an open footbox to a closed one increases the weight by 1 oz. In contrast, the EE Enigma with 10d shells is 19 oz, but it seems to be 3″ shorter and 1″ less in width and have less weight in higher quality down, so in real life is probably comparable. EE allows you to increase the top shell to a 20d, and this increases the weight by 2 oz. The cost difference is in the $75-100.
Back to your original question, my guess is that the saver series Ion shells add about 4-5 oz of weight to a quilt over 10 d shells . You are the one to answer whether that is worth $100. I personally do not think there is much difference between 800 and 850 fill, but I am no expert.
For reference, another manufacturer that offers a good price point is Underground Quilts, but note that their down is not DWR treated https://ugqoutdoor.com/top-quilts/