Hecarim gains AD based on his movement speed. Is it worth the investment of ghost over flash when jungling Hecarim based purely on the AD gained (ignoring the benefits of flashing over walls etc). So, how much damage is gained at each common movement tier (Level 1 boots, Level 2 boots, Boots of Mobility, etc)?
Ghost, like most movement speed modifiers, is a percentage movement speed modifier. Percentage modifiers are accounted for after flat movement speed modifiers. Flat movement speed modifiers are typically specific champion abilities and boots.
As boots are flat movement speed modifiers, I will account for those first. As Hecarim's passive scales up as he levels, I will use level 18 as a baseline (25% movement speed to attack damage)
Ghost provides 27% increased movement speed (35% if you take the Summoner's Wrath mastery in the Offense Tree). To save space, I will calculate the effectiveness of Ghost using Hecarim's base movement speed (345).
Note that this difference in attack damage will become more pronounced as you gain flat movement speed modifiers (such as boots or being the target of certain champion abilities).
From this, we can see that Ghost provides a significant increase in movement speed, which will give Hecarim some more bite on his abiltiies. But this also means that Hecarim shouldn't use Ghost like most champions. Rather than using Ghost for initiation or escaping, use Ghost as a damage steroid. Hecarim is already an impressive initiator without using Ghost.
*Note that the movement speed buff is reduced to 45 upon dealing damage, so the extra attack damage is only effectively for the first hit.