I'd call the above a description of viable for progression. If you can easily keep up with the people you run with and your group isn't extra-frustrated that they (you all) don't have a bit more DPS to down a new boss, sub is viable for what you are doing. If you're taking progression seriously (at a non-heroic pace at this point), you *could* use Sub without falling behind, but it will make you work MUCH harder for the same damage, survival, and usability - averaged across all encounters. Some are less punishing than others.