ORIGINAL: NoxNoctum
Does this have any affect at all on rate of fire?
It does, indirectly in two ways. On the one hand, it makes the unit more likely to engage enemy units which are deemed to be direct threats. On the other hand, it makes the unit less worried about target overkill while firing.
One important point which I reckon it's not very clearly made in the manual, is that task aggressiveness doesn't "override" other factors we use to determine "unit aggressiveness". It actually has the same weight as:
[*] Unit Morale, Cohesion and Aggressiveness rating
[*] Commander Determination, Aggressiveness, and Determination ratings
That is, if you have three units, A, B, and C, commanded by equally "wimpy" officers, and all other relevant ratings equal, each unit with tasks set to "Min", "Normal" and "Max", what you'll observe is that C is less "wimpy" than B, B is "wimpy" and A is more "wimpy" than either B and C.
It's also important that "unit aggressiveness" is the main parameter when it comes for a unit to decide whether to close the range with the enemy, and fight through their positions, or stay put at 'standoff range' - that is, having the enemy within effective range of its most lethal weapons.
ORIGINAL: NoxNoctum
For example, in a situation I'm in now, I'm ordering a Pak40 unit up to help defend a town. Now I want that Pak40 to shoot as much as possible, but I don't want it to engage anything on its way to moving up to its defensive position---if I set aggro to minimum, and don't bother to change it once it's arrived at its destination, will it still fire as much as possible at enemy targets if I have the ROF set to max?
No. This can be a bit confusing so bear with me.
When you issue a "Defend" Task, this will spawn a Move task, which will get your unit to the objective location. The settings for this subsidiary task - that achieves the precondition for the defend task, namely, that you're at the objective location - are derived from those of the Defend Task. This Move task settings will be derived from those of the Defend task. So in the case above, if you set the Defend task settings to Max Aggro, the settings for the Move task will be max aggro as well. You can check this yourself, by inspecting the sub-task icons (i.e. task icons with a grey background) that should appear while this Move task is being executed.
When you issue a "Move" Task, a Defend task will be generated automatically at the destination, with the same general guidelines above applying to settings.
In this particular example, I'd suggest that you issue first a Move task with min aggro (and the bypass setting checked on) first. This is because as soon as this task finishes - your AT platoon or battery reaches its destination, you'll get a message on the UI, so you can immediately issue a Defend task.
This is an instance of micro-management which we want to get rid of, ASAP, by allowing the player to issue 'sequences' of tasks, so you can control explicitly the settings of each of them.