Let's compare his old passive damage versus his new passive damage
Level 3:
NEW: Tahm has 750 health at level 3, 810 if you start with a Doran's Ring (I always do for mana regen, mostly). So 1% of that is 8.1. So if you decide to all-in at level 3 in a 1v1 situation, that's Q-auto-auto-W-auto-auto-Q in that approximate order (other combos exist, but we'll go with that for now). So we take into account the passive stacking, and we deal 145.8 pre-resistance magic damage over the course of 7 damaging attacks (somewhere around 100-110ish total applied damage). Which isn't bad by any means for level 3, but it's not enough to completely bully someone out, especially considering that you'll use about 200-250 mana at level 3 for that fight, which is about half your mana bar.
Level 6:
NEW: Tahm at level 6 has 985 base health. If you have a Bami's Cinder with the Doran's Ring listed above as an example item build for this stage in the game, then you'll have 1,325 health. So 1% of that is 13.25 health. Q-auto-auto-W-auto-auto-Q now does 238.5 magic damage, pre-resistance.
OLD: The old passive gave Tahm 20+2% bonus health magic damage every damaging attack or ability. So with the same build above, that's 20+6.8 (2% of 340). So the same all-in combo gives him 186.7 magic damage. So the new passive only gives him 50.9 additional magic damage across 7 damaging attacks. Which is a little bit extra, but it's not much.
Level 11: Now let's assume you have a Sunfire, a Spirit Visage, and a Kindlegem for your Warmogs, and you've sold the Doran's. That's 1,200 bonus health, and his base health is 1,444, for a total of 2,644 total health.
NEW: The passive now deals 1.25% max health, which is 33.05 (we'll say 33 even for simplicity and cleaner numbers). So the same combo above now deals 594 pre-resistance magic damage over the course of 7 attacks, for an average of 84 magic damage per attack
OLD: 20+4% bonus health (48). Over the 7 attack combo, that's 476 damage, so the new one gives him an additional 118 damage over 7 attacks, which is an additional 16 damage per attack.
Level 18, assuming full build: Here's where the big difference is. Sunfire Cape, Spirit Visage, Warmogs, Dead Man's Plate, and Titanic Hydra with an Elixir of Iron. 3,100 bonus health, 2225 base health, 5,325 total health for our example end-game build.
NEW: 1.5% of max health (79.875), stacking. Again with the Q-auto-auto-W-auto-auto-Q combo, that's 1,437.75 damage.
OLD: 20+6% of bonus health (186), over 7 attacks that's 1,442 damage, which is essentially the same as the new passive. HOWEVER, if we break this all down over shorter combos (because how often outside of laning do you actually get to attack the same target 7 times in a row?), we get a very different picture. Over a 3 hit combo (Q-auto-auto), we get 618 damage for the old passive and only 479 for the new passive. So if you attack 3 different targets 3 times each in an extended team fight while you frontline for your team, then you're going to deal a total of 417 less damage in that team fight.
Furthermore, the new passive needs to stack to reach full damage, while the old passive was flat damage that was immediately applied. EVERY SINGLE SCENARIO WE JUST COVERED WITH THE 7 ATTACK COMBO IS IDEAL FOR THE NEW PASSIVE, AND IT STILL BARELY OUTPERFORMS THE OLD PASSIVE FROM LEVEL 6 TO LEVEL 16, AND ONLY MATCHES IT IN THE LATE GAME. So a single Q now deals 126 less damage with the new passive than it did with the old, which is about 8% of an ADC's max health in relative damage. Similarly, a 2 attack combo like Q-auto now deals 240 additional magic damage in our end-game build scenario, whereas it would have dealt 412 magic damage with the old passive. So in the end game, THE ONLY time the new passive deals the same amount of damage as the old passive is after 7 CONSECUTIVE attacks against the same target, which will almost NEVER happen.
So, I'll reiterate. The new passive gives him a moderately stronger early game, but it isn't enough to turn him into the lane bully he needs to be in order to make up for the heavy damage nerf to his late game.
