I do not know about "other" improvements on the AI part, but this was actually posted by a CA representative:

1) on hard, the AI gets +4 attack bonus (no defense bonus)
2) on very-hard, the AI gets +7 attack bonus (no defense bonus)

there are also unspecified morale bonuses to the AI side.

as to the strategic map: the only difference i've seen is that rebels tend to pop up more often than in normal. also, some people are suggesting, auto-resolve uses "boosted AI attack" in calculation. even for sea battles.