The first thing Hardcoreness can buy is this one.
The second Hardcoreness bonus is extremely profitable once you begin gaining lots of XP.
The third Hardcoreness bonus becomes pretty much necessary if you want to get to 100 Mastery at all.
Finally, the Rebirth button is available.
Playing anew after a Rebirth. Only one? Not enough. I think I'll do it 99 times more.