Shouldn't Thyrus+1 be the last item I get when it comes to using Myth tomes?

[Airikay]

If people keep saying 10 or 30 isn't much of an upgrade it ends up adding to "o few hundred a heal doesnt matter". The +1 is the greatest % increase per tome, so it's the best.

[Menae]

If people keep saying 10 or 30 isn't much of an upgrade it ends up adding to "o few hundred a heal doesnt matter". The +1 is the greatest % increase per tome, so it's the best.

Unless you are somehow able to equip 7 sets of gear and bank 6300 myth, it really doesn't. There's only four ways in the game to improve your stats:

Gear - by far the largest, but it takes a long time to increase . Assuming you don't buy relic +1, you will be 30-40 per Cure behind less whatever the other pieces you buy provide; let's assume you buy NOTHING and just let your tomes rot.
Food - largely irrelevant. You can't add Mind via food, so the effect is pretty limited. Maybe 3-5 per Cure if you are using HQ Det food (caps at 17 Det, 9 crit if you have at least 425 Det/450 crit via using HQ Black Truffle Risotto).
Potions - on a long cooldown. Useful to have for burst healing (61 mind for HQ Mega Potions of Mind), but the long CD makes them much less useful than they are for DPS. Difference between using HQ Megas on CD vs. using none at all is, averaged over time, about 3.75 mind, so another 5 or so per Cure.
Stat allocations - about as powerful as upgrading from relic to relic +1. Up to 30 mind; so another 35 or so per Cure (vs. allocating for Piety or Vit).

If you decide, for whatever reason, to choose the "wrong" option for all of these, you will lose about 85 per Cure, and, what, 130 per Cure II? Potentially in exchange for the money savings of not using potions (they aren't very good for healers), not using food (also not great for healers), the option to buy better looking gear or skip out on some of the darklight grind, and potentially having 30 vit or piety. That's a maximum of about a 10% healing increase you are giving up, true, but who runs with 30 vit/piety anyway? I personally tend to min/max, but there comes a point where min/max isn't really worth it. For some it's not slotting their crafted jewelry with 5 level 4 materia. For some it's not using food/potions. For some it's sacrificing Thyrus+1 for a few weeks to get something else they'd rather have, whether it's for looks or whatever.

My personal min/max threshold is at food. I have +1, I have 30 mind statted, and I use food, but not the best stuff; just some cheaper stuff that costs me 2 vit and 2 piety but costs 1/4 the price. I don't use Mind potions because they are too expensive and not helpful for our raid's issues - when our tank dies, it's not because I'm not healing for enough, it's because we don't kill conflags in time, or he stood in Divebombs, or other mechanical issues that aren't stat-based. I'm considering buying the Cleric's robe if the Allagan hat drops because Darklight looks terrible, despite it not being my optimal upgrade path, because I'm geared enough to down all the content in the game right now, and further upgrades won't make anything much smoother. Gear just isn't that strong a force for healers due to the nature of overheal and fights which feature mechanics that aren't really survivable if you screw them up.