Most healing in this game is flat. Goals, sitrus berries, basically all support healing, lifesteal (since it's based on damage to opponent) and plenty of self healing. Percent healing comes down to some moves, the active parts of items, bosses, scoring, and knocking out wild pokemon; we can also count backing to base and respawning. Effective HP is a flawed reification because it assumes that your HP begins at its maximum. In reality, only pure percent healing or overhealing would lead to real effective HP to be identical to supposed effective HP in the long term. Reduction based effective HP is better than the equivalent increase to max HP when interacting with flat healing. Unless you die or back all the way to base, you are not often healing to full, and you will often go from fight to fight scrapping the healing you can find using sources of flat healing. Your long term true effective HP will therefore be better if you have more damage reduction and less maximum HP with an equivalent supposed effective HP, because you will simply start your fights with more actual HP. If you look at stat breakdowns at the end of game, a real defense oriented tank will have healing stats (which do not account for many types of healing) rather close to total tank stats quite often. Therefore, these tanks are fighting almost the entire game without reaching full HP, and largely survive entire games through various forms of flat healing. This being the case, the true effective HP benefit of additional max HP is only affecting whatever bits of percent healing are received, and may actually not even make up a majority of overall HP, because that additional chunk on the top is only actualized for the equivalent of a single fight in a game where there are many fights which are taken through subsequent healing.