Mounted units already do more damage in open terrain; if more flat/rough difference is needed then maybe the rough terrain defense boost should be adjusted instead. Mounted units already gain no defense boosts from terrain. All that rough does for them is make movement slower and reduce their damage. I thought the point of raising moves to 5 was to eliminate the 1 benefit of rough for skirmishers: using obstructions to attack without reprisal. With 5 moves that can attack/retreat without reprisal as well, so the rough terrain bonus is nullified. After that, I see any bonus to rough/flat as redundant to the systems that already incentivize mounted units onto flat.