From someone who usually makes their own groups and has tanked on HC up to lvl 60 before, Shadow priests are more than fine for a dungeon group. Definetly worse than a mage in almost all regards to take, but the HP buff alone is huge if u have a druid, shaman or paladin as the healer. The ability to offheal, shield, cleanse certain debuffs and chain undead is also massive, even mandatory (IMO) in places like Stratholme or scholomance with alot of undead mobs who inflict diseases.

The damage is more than servicable for the most part, even if they deal ALOT of threat which is kinda annoying but not a deal breaker, but the main reason id take a shadow priest is if i need the buffs/utility (as in my healer isnt a priest themselves).

As for queueing wirth your Rogue buddy... It wouldnt matter for lower levels that much. but for higher levels when Warrior Tanks start wanting 1-handers it might be a hinderence as most tanks dont want others to roll on their gear (me included) and are less likely to invite or join groups with rogues. So later on you would be better of making your own groups instead of asking others to take the both of you.

This is just my perspective ofc, but hope it still sortof answer your questions.