Assuming one divine and one arcane healer, the difference between a 0 mana pot, smooth normal ToD+suulo and a 5-10 mana pot one is the healing amp of the Horoth tank.
A tank with Leviks and Human Improved Recovery 3 is *always* a smooth run, even if DPS is terrible, as (outside of a Disintegrate failed save) they can be healed from nothing but scrolls, letting you use your Heal cooldowns and SP to prevent party members dying to trash. I'm happy to soloheal a ToD like that if an arcane will babysit the Suulo tank, and the best ToD tank I've ever run with (and although he's not a member, he's our guild's first choice on the server for Elite towers) is an only moderately geared human barb (no Bloodstone, only one Shroud accessory) with ~71% total healing amp (10/20 on DT, 30 from Human racial enhancements).
A tank with no healing amp cannot be healed, even on Normal, with just one healer's scrolls (the scrolls just keep pace with the badges), so even an exhausted Horoth will require SP and Heal cooldowns to be spent on the tank.
Of course some of that healing is overhealing - but if you have a tank that's at 470 of 690hp, would you rather your scroll hit them for 132 or 264? Or if they are at 200 of 690hp, would you rather your Heal hit them for 285 (leaving them still at risk of instant Disintegrate death on Horoth), or for 570?
In both cases, give me the overhealing any day of the week. And as for past life passives, give me a tank with something that makes a real difference in the Horoth encounter - 5-15% healing amp - over one that has 1-3 extra damage per hit, which in a three minute fight equates to 420-1260 damage - 0.21-0.63% of Horoth's hitpoints on Normal, and 0.12-0.36% of the combined HP of Horoth and Suulo - shaving, in the best case, about 0.5 seconds off your completion time.