IMO its fairly straightforward. Morality requires intelligence just as construction of buildings that do not fail requires intelligence. To decide on an action based on some high level moral imperative, one needs to think a fair lot.

Most people are just too stupid to be moral. They were with their own hands murdering minorities if they were in right position in 3rd reich. They were burning witches. They aren't even facing a choice to be moral or not. They are hundred percent amoral as far as big picture goes. They need to obey very direct simple rules made by others, the end result might be moral-ish for good set of rules. They can't reason from their action to any high level moral imperative. In a discussion they'll say that you can't either. Hell they'll say it with emphasis, seeing it as virtue - the 'its wrong' kind of can't.

The intelligent people... A normal kid who's grown with presumption of mental disability, among mentally disabled, will play mentally disabled to get slack. So do many intelligent people grow up with such habit, spoiled by having plausible deniability of intent via being stupid.

In light of this I think that intelligence enhancement, in culture that does progress towards improved morality, would improve morality.