Sorry about the late reply! The Nav Modifier volumes should essentially ‘bake’ the specified Area Class (via Area IDs) into the polys. However, I also expect that the volume will force extra polys to be made to clearly distinguish areas with different area classes (may need clarification on this, but it seems like a sane assumption). This would definitely have an effect on performance as there are more polys to search through, but I envision it wouldn’t be too bad - you can profile it by using the stat Navigation command in console and compare the baseline with the added volumes.
Yes, they’re similar solutions, just that Solution #2 allows you to write your own algorithm on path building. SetAreaCost is for the duration of the area and SetFixedAreaEnteringCost is for the initial cost of entering the area… imagine if those two are added, you have your total cost of traversing that area. If you’re wanting to modify path costs dynamically, you will have to write your own FindPath algorithm that queries the Detour system (like how you’ve started in your other question).