Not to my knowledge
"for DAS games where the EV to double down on 4-4 vs 5 or 6 begins to exceed that of splitting them (like some very high index number)?"
Don't think so. Logically, neither 8 nor 4 is a thrilling starting hand, so the main angle is hoping that the dealer will bust the 6. Splitting has too much appeal, in my mind, because of the multiple possible draws (4, 5, 6, 7, A) that will lead to third, fourth, or more bets.
But, this is my intuition, and, whereas that has a very fine track record, if someone can crunch the numbers, that would be good. I don't think that the doubling e.v. will surpass that of splitting, regardless of index.
Don