Although there is a difference in the house edge between 6 and 8 decks, it is very small; even Stanford Wong points out on p. 36 of "Basic Blackjack" that playing against more than 4 decks is worse than playing against 4, but only by a small amount. The difference between 1 and 2 decks is far more significant than the difference between 6 and 8. I used the house edge calculator on Michael Shackleford's site (which I've linked in this post) to come up with the following house edge figures for games with S17, DA2, no DAS, split to 4 hands, no LS:
--1 deck: 0 ==> BS player should break even (over long-run)
--2 decks: .336% ==> BS player should lose $3.36 per $1,000 wagered
--4 decks: .495% ==> BS player should lose $4.95 per $1,000 wagered
--6 decks: .548% ==> BS player should lose $5.48 per $1,000 wagered
--8 decks: .574% ==> BS player should lose $5.74 per $1,000 wagered
Thus, the difference between 6 and 8 decks only amounts to $0.26 per $1,000 wagered, whereas the difference between 1 and 2 decks amounts to $3.36 per $1,000 wagered. My point: for the BS player, the difference between 6 and 8 decks is rather insignificant when you look at the "big picture." Another way of looking at it is that an 8 deck game with S17 is better than a 6 deck game with H17 by approximately $1.90 per $1,000 wagered, if all other rules are the same; in fact, 8D-S17 is better than 4D-H17 by about $1.36 per $1,000 wagered. The bottom line is that rule variations such as S17 vs. H17 and DAS vs. no DAS are much more important to the BS player than 6D vs. 8D.
