FYI the Pathfinding behavior uses the A* pathfinding algorithm on a grid. It's also massively more efficient in the C3 runtime.
Generally so long as you have a reasonably large cell size and you're not using ridiculously huge layouts, it's pretty fast. The main cases people find it's slow is if they set a tiny cell size like 1x1, which forces the pathfinding algorithm to search through tens of millions of cells, which is really slow. The whole point of using a grid is to massively speed up the search by reducing the amount of searching that needs to be done.
1. Does simply having Pathfinding assigned as a behavior to an object (enabled, but not actively being used) cause any considerable demand on CPU?
No. It's only finding a path that uses CPU.
2. Would destroying an instance of an object make a difference as soon as it's not needed?
No, because of 1).
3. Is a grid generated per object using Pathfinding?
Grids are shared between all instances, but if you change the cell or border size, it has to create a new grid out of necessity. But you should not depend on that, it's designed to share the grid between all instances to make it much more efficient on CPU usage and memory.