While true, do remember that 5% isn't all that much these days. Since baseline is now 45% (your Eidolon) + 45% (friend Eidolon) + 45% (Elemental advantage) + 30% (Relic weapon) + 100% base, the difference is... a little less than 2% per Ilu. Since not many others are running Illu, the question is, does a full Water grid give a 2% base Atk output loss or not?
I haven't done the maths, but 2x LMB Tiamat is probably a requirement for that to work out for you. Overall, I doubt it's worth it - just run Reiki/Tiamat/Jack Frost depending on what you need.