Here you go!
Marco,
Naturally, the precise answer depends on a slew of variables, so I made several assumptions here: heads-up HiLo counter using full indexes and spreading $10-$120 (in $10 increments) for a H17 DAS game.
For 6D with 78 cards cut off, the win rate is $17.55. The 8D game with only 68 cards cut off (so 6.69/8 pen, or 84%) has almost the same WR: $17.56.
Note, though, that only the WR's match: the CVCX optimal bet schedule for the 6D game is $10 at 0 & below, $20 at +1, $70 at +2, and $120 at +3 & up; for the 8D game, the bet schedule is nearly the same, except that it has no $20 bet but instead stays at $10 at +1, then jumps directly to $70 at +2.
This difference in bet schedule makes the average bet lower in the 8D case, $23.74, than in the 6D case, $25.31. This means that the 8D game has a higher SCORE than the 6D game: 13.32 vs. 12.73.
To generalize, bigplayer has stated that two games with the same rules but differing numbers of decks will have about the same EV if they have about the same number of cards cut off, which is a pretty good rule of thumb.
Hope this helps!
Dog Hand