Assault doesn't matter here. Increasing Elemental by 5% (total) is going increase you damage output by 5%, regardless of your Assault and base Atk values. Of course, the higher those two are the higher that 5% actually IS, but the ratio stays the same.... because there's a difference between 5% more to 165 (8.25) and 265 (13.25). That's why it matters when you're calculating the difference between two values.Uuh, I still recommend to FIRST focus on maxing one Grid, but I don't think I've ever said to ONLY max one Grid? That'd be sanathlig I'd think, who wanted to complete all events with minimum amount of effort required.
Anyway, if you want to compare without Elemental advantage... then let's go with Tiamat as well. So it's 40% (your Eidolon) + 40% (friend Eidolon) + 30% (Relic weapon) + 100% base = 210%. A 10% to that is ~5% increase, so at best, Ilu is 5% more damage (each). But do keep in mind that your burst generation will quite a bit lower that way, when compared to double Tiamats. So for a 40k base Atk player, that's about the same as 2k more base Atk. Chances are very high that you can run a full Water Eidolon grid and not lose 2k base attack, and thus it's a damage increase. And that's just with one Ilu, if you find another one from your Friend List then it'll absolutely be worth it (outside of Elemental advantage).
Still, considering how situational it is, I'd still say to not bother with it.