What about the third option: increase boss hp and damage and introduce mechanics from Naxxramas.
Players want a challenge: this realm is supposed to be hardcore. So make hardcore raids. Adding mechanics still makes the bosses easy to kill. It just adds complexity that more players will need to be aware of.
Adding both, however, means grouping with people who can interpret tactics and those who are willing to sacrifice a portion of their time and become devoted to raid with a hardcore realm. Increasing the health and damage means that players will have to get better gear, learn their class, gem and enchant their gear, gather resources such as potions to make sure they are at their best, as with this addition, every dps and heal should count to determine if you are successful in the fight, so they will have to be committed to achieving a raid kill, rather than killing a boss with mechanics and learning the added complexity. The added challenge of both features would emphasise the hardcore nature of Lordaeron and the commitment of players who want to achieve a greater challenge.
-- Update --
New Idea: I think the added health, damage and/or mechanics should remain until the next tier of content has been released. However, perhaps there is a limit to when it should be eventually removed. Current content should be challenging and those playing on Lordaeron should earn their gear. There should be a reason to play on Lordaeron, and excluding the fixes to Lordaeron, the challenge would otherwise identical to those at Ragnaros and Deathwing without the added challenge. So why make an identical server when people can go play on those casual realms should they have enough of hardcore content at Lordaeron.