It's hard to tell for sure, but since you get a 98% damage reduction after 10 levels in legacy raids, I took 200-250 mainhand and 125-150 offhand from Illidan. So 10k-12k mainhand, 6k-8k offhand attacks on mail from him.
One super interesting things here is that the raid MIGHT get the scaling technology for raid size. I was watching Towelilee's stream and he had a mostly full raid and the bosses had 2-2.5x the hp of what they even had running it solo. I don't want to get too excited, but you might be able to run BT with Warlords/Legion raid scaling at level 70.