Different roles will have a different hard cap of justice points per week.
Example:
Dps - caps at X points per week divided by three
Tanks and heals - caps at X points per week
This does not mean that tanks and heals will be able to get gear faster than dps because the cost for dps gear will also scale to cost less. Dps will simply be discouraged to enter the queue if they have hit their cap in less runs, making the queue faster because less dps will be in the queue overall.
Regarding dual spec, or if your tank/heals drop and you swith r