I leveled my 5-man almost exclusively in instances from 20ish to 60ish. Very fast. It slows down at 60-70.

I took instance breaks to do pre-reqs for instance quests. Also went to The Hinterlands and did the bunch of quests in the elite troll city there (Jintha'Alor?) as you get quite a lot of quests for it and with a 5-man it can be knocked off very quickly for mega xp and some decent trinkets.