I had this same problem. Trying to drill the broken bolt perfectly center with the head still on the engine is very difficult. I tried this and ended up drilling slightly off center and breaking the easy-out tip in the hole . I was beyond pissed! So I removed the head and took it to a machinist buddy and he got it out no problem. When I went to go pick it up, he said," If you would have just removed the head to extract the bolt, you wouldn't have drilled the hole off center, and that would have meant that you wouldn't have had to leave the head with me for a couple days. Besides, you had to pull the head anyway right?"
With that being said, if it ever happens to me again I'm going to pull the head and do it right. I'm not saying that it cant be done, but I am saying it is'nt easy. Especially when its the the stud closest to the firewall. Pulling the head isn't very difficult at all. The hardest and most time consuming part was cleaning the head and block for reassembly. Good luck!