There are darts on the inside shell but those particular darts are a result of compound differential curves which could have been achieved without darts on the inside shell if I had chosen to put two darts on the outside shell for every one of those darts on the inside shell. If I understand what you are asking, the inside shell is taut and smooth, like the exaggerated differential curve on the sketches I posted as opposed to a reverse differential curve. Your draft stoppers you talked about on one of your underquilts are kind of like a reverse differential curve. Reverse differential curves are good for filling in gaps and are used in the hoods of some sleeping bags.
The "negative curve" can still leave an air pocket with the SnugFit but the full width suspension system helps minimize that and it isn't very noticeable. If the suspension system doesn't pull along the center area of the underquilt, that "negative curve" will be more problematic, particularly if you lay a certain way. I used a net hammock to easily test for that for gathered end hammocks when I was testing the suspension system. When I was testing asymmetrical hammocks I just had to feel around with someone laying in the hammock and it seemed to work fine on them also.