Number of decks used is misleading. The number representing the change in house advantage appears to be pretty minor, but it is really much greater.
Take 6 deck vs 8 decks for example. The differential in house edge is .03, a seemingly small and almost insignificant difference But the difference in win rate is vast. The BIG difference is when you break down true count frequencies. Night and day!
It took me 5 years to see this, actually. It wasn't until I moved to Vegas and had access to mostly 6 deck games as opposed to the 8 deck games that I played in Atlantic City my first 5 years that I understood just how great a difference it is.
And double deck compared to shoe is even more dramatic. However Double deck comes with a whole other set of issues on the negative side (heat) that have to be weighed into the equation.