The dots are helpful but kinda overrated IMO. Even an arcane without dots can do roughly 2x the dps of an equally-geared melee and sustain it for the duration of a boss fight. From my experience, the reason arcane slots are limited in eDQ is not because of the damage loss but because the server is teeming with 300-hp sorcs that want to take a shot at it, and people are trying to be prudent about not losing too many people while doing the raid.
Actually, it is more likely that failure would be in P1 where half the party will fall asleep waiting for the portals to go down. All the other parts will probably be caek. Doubt Harry will last longer than 30 secs in P4.
Are you sure about this? IIRC elemental spells don't work on Xyzzy.
Cheers.