5x mage will be tough in instances, no healer and no rezzer. Aside from that you do have an insance amount of CC. If you can get the CC to work well for you and consitent you shouldn't have problems. But I wouldn't recomend DM untill 18-20 w/o a tank or healer.

As to quest vs. instance. I think your team would have a harder time with instances as to the reason I alluded to above. All aside questing is the way to go if your not boosting.