Im testing a 15 land set-up right now:
4 City of Brass
4 Gemstone Mine
4 Cephalid Coliseum
2 Tarnished Citadel
1 Undiscovered Paradise
I definitely believe that at least 2 Tarnished Citadels is the right way to go. I hate the damage sometimes too, but it's a necessary evil. On the other hand, I think 3 Citadels might be the threshold for too much damage, which is why Im running Undiscovered Paradise over the 3rd.

