I am unsure, but if I were to guess, I would agree that the index should be slightly lower for hand totals of 16 with more than 3 cards, although for completely different reasons, as yours is voodoo
The index for 16vX, from what I understand, is created using different combinations of 16 (including 2+ cards) and then evaluating the EV of hitting/standing.
The difference is that 16s with 3+ cards are much more likely to contain small cards as Aslan has said. However, the difference is that instead of considering streaks, we consider composition. The deck itself has fewer cards that improve our hand compared to a deck where we were given a 2 card 16. This means that we should be more inclined on standing, at least when the TC is = to the index. However, if the TC is something like -.2, is that TC enough to deem a stand? I would hit, as I don't know the answer, but I wouldn't be too surprised if such were the case.
The same could be said for any multicard hand total v any card for their appropriate indexes (perhaps except for dealer aces, as they are stronger with lots of small cards after checking for BJ).