Sadly tbc instance boosting after lvl 70 is pretty bad. Blizzard made it so that you take only 10% of XP. It is to discourage people from storming old content instead of leveling in the new one. I didnt try going there without my main lvl 85, so maybe if whole group is 70+-, you would get normal XP.

I am leveling 2 new warlocks now in UK and I am getting 700-900 xp/hour depending if following alts die a lot But I am considering doing quests again, which I realy hate to do, but 30k XP per quest is something to not leting go away.