Yes it can reach 100%, double the performance.
Since the rise of the HD 4000 series, scaling became as good as 50% increase and maybe more.
HD 5000 series, scaled well up to 86%. HD 6000 series including HD 6950/6970 reached 100% in some games.
So most of the latest generations from AMD & nVidia scaled well, depending on the drivers, games that support CF/SLI.