You don't need to compare two different attempts to understand the defensive trade-off. You just need one. The vast majority of the damage in A9S comes from autos, with intermittent damage spikes from Scrap cleaves.
Autos hit for about 6k without Grit. You would have mitigated about 1.2k by being in Grit. The numbers are actually quite similar between the two runs if you calculate what Grit's contribution is in each case. The big difference between the runs is the number of autos received: 63 in the faster run, and 95 in the slower one.
It's a similar story for Scrap: you take 8k without Grit, and mitigate 2k by being in Grit. The difference again is that the faster attempt only sees half the number of cleaves as the slow one. It's worth noting that the upper bound damage for Scrap is about 10k without Grit, which is important for both you and your healer to gauge what the minimum safe threshold is before intervening.
I'm not really sure where you got the idea of the "probable"(?) soaked Double Scrapline from. You can directly check.
Damage Taken Per Second is not particularly useful here. Damage occurs at very discrete intervals, as does healing. The difference in damage taken with or without Grit is only relevant if it forces an additional heal or if it results in a death that could not have otherwise occurred. A 2k difference gets swept up in a Regen tick, and we're looking at max HP in the 32-34k range.
Turning off Grit is more than a 25% dps increase for a couple reasons. First, you have access to BW, which influences both CPM and available MP, both of which directly translate into dps. Second, as the raid encounter time drops, you spend more time in your opening burst. Even if you could keep all other external factors constant (i.e. party comp, buff/debuff uptime, proc and crit rates for all players), this second point is something that you can't control for. Clearing more efficiently will "inflate" your dps significantly. And even if it was only a 25% boost ("only" being more powerful than a permanent RS or Balance): why give yourself a dps penalty needlessly?