Water Treatment & Mash Efficiency

[WishboneBrewery]

Is there a direct connection between correct water treatment and the efficiency of the mash?

On my last Stout I got 94% Mash efficiency, whereas Yesterday's Belgian Brew I only gained 76%.
The main differences would have to be the Stout's liquor was treated to the GW Calc's Stout setting and my Belgian simply had 1 Tsp of Gypsum to the Mash.

Any correlation here?

Also, does the Mash Temp alter the Efficiency, so 65c for a dry Beer would be more efficient than a 68c mash for a fuller bodied beer?

[WishboneBrewery]

Obvious really I suppose, I just don't normally bother... Cheers