I've fought seafuries since I was 38th level. I was also the highest level person in the group. They have no where near 5K HP -- any of them. At most they have 3K and I'd guess maybe a little less than that.
As for XP, I can gain a bubble of XP in 40th level hell in about 4 hours killing these guys. Maybe not powerleveling, but still a good rate and is pretty safe if you have someone that can pull safely.