New to 5-boxing, but seeking advice about 1 of my 5's builds

I finally broke down and upgraded to 5 accounts. :D
I've done 2-3 boxing for a while, so I'm not entirely a n00b.

This will be my first attempt at a variety of classes though.
I've made up my mind to go with 1 x Prot Pally, 1 x Resto Shammy, 1 x Affliction Warlock (with imp), 1 x Demonology Warlock (with succubus or fel guard) and 1 x Mage.

I don't have much experience with Mages. I do know how to make a decent solo Mage that I would probably enjoy playing.
But I would rather have a Mage that would compliment the group.

Ok...my question:
Right now I'm leaning towards the Arcane talent tree to get Slow. Is this worth it in a 5-boxing team, or should I go with something else for more DPS instead?

I have crazy ideas for this group, so I probably won't be talked out of it's members, but any advice is welcome!

Thanks!

I would go full DPS Deep Fire for the Mage.

Mage's role in a 5-man would be DPS and sheeping.

Thanks for the advice. :)

My idea is you will be casting every 1.5-2.5 sec on your mage, but others will just be most auto-attacking. That is tricky on my part since I am used to just mashing my attack macro about 2-3 times total. For the slow spell, it isn't going to be until lvl 51. That is a LONG time until then. Use whatever you think will help until you get there then you can respec if you want to get it. My 2cents. You have locks for anti-runners, so slowing mobs with ice isn't that important. You probably don't want your mage to get aggro so Blizzarding is probably out. I might chose arcane for arcane missles since it is channeled and it will auto-follow mobs incase they move around. Nothing like a "you aren't in facing the mob" to piss you off.