And I'm not ignoring anything - the fact is, the more spirit you have, the more you can throw out expensive spells and do bigger heals. The difference in the different spells' power is so huge, that no matter of int could be better than simply choosing the next spell on the ladder if you need more throughput. I don't see what the issue with not seeing the benefit of your playstyle till the last 90% is - no one will care if you're healing targets for 1K more per cast for the first 90% of the fight, if you're OOM for the last 10% and become a burden to the other healers.
There's 3 fights this tier where I'd prefer int to spirit, and that's tsulong/stone guards/garajal. Two of these bosses are among the easiest of the tier. The last one, as you're a resto druid, you probably won't be allowed to soak up light of day buffs, meaning you might aswell not bother with the extra int, and make sure you keep the raid alive in Night-phases, so the hero-healers with proper cooldowns can shine in the day phases.
I'm not sure I fully understand the second part of your post, though - can you elaborate what you mean that I can accumulate extra healing to get a significant difference in terms of HP? Do you want like, a "scenario of healing", such as -
Over 150 seconds you cast 20 healing touch, 20 rejuv, 10 swiftmend, 15 wild growth
And from there on calculate how much extra healing the 500 int would give? Or am I misunderstanding?