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The "normal" infantry model, which includes conscripts, are sub type: Regular, family: Line, Move Type: HvyFoot, Move Rate: 100.
Light Infantry on the other hand have sub type: Irregular, family: LightInf, Move Type: MedFoot, Move Rate: 110.
To compair
early CSA cavalry has sub type: Cavalry, family: Cavalry, Move Type: MedHorse, Move Rate: 130
late CSA cavalry has sub type: Cavalry, family: Cavalry, Move Type: MedHorse, Move Rate: 120
early USA cavalry has sub type: Cavalry, family: Cavalry, Move Type: MedHorse, Move Rate: 100
late USA cavalry has sub type: Cavalry, family: Cavalry, Move Type: MedHorse, Move Rate: 100
Since I know that even though both the early and late war USA cavalry have a Move Rate of 100 they are still much quicker than infantry, I will assume 2 things:
1) the Move Types have their own factors, Hvy and Light, Foot and Horse, and
2) the Move Rate modifies these
So normal Line infantry build the average marching speed being HvyFoot with Rate 100. LightInf are somewhat quicker with a Rate of 110.
What this doesn't tell us is what the actual difference between Foot and Horse is.
But Pocus as you stated, it also depends on the terrain and since LightInf are Irregular they should have an advantage in rough terrain like forest, mountain and swamp when it comes to frontage.
If and how this plays any role in defending against being pursued I can see no reference from the numbers in the models. The closest that I could guess at would be the sub type Irregular, but that is very speculative.
My summery wold be that LightInf have an advantage in frontage in non-open terrain, but since LightInf only come in brigades with at least 2 other HvyInfantry regiments it would take a lot of effort and coordination to build for example divisions with the largest possible number of LightInf regiments (you might be able to squeeze 4 lt, inf, inf brigades into a division, but at a cost of having too little room for other unit types, so 3 would be the practical max) and put these into a corp for use in rough terrain, but I don't see the Bang-For-The-Buck™ in the effort.