Hmm. If it's 5%, then average attacks goes from 1.14 to 1.24 per turn (from double Tiamat). That's a... ~9% damage increase before we consider synergy with burst generation. The biggest problem here is that if the Soul takes no damage, the amount of turns that you need on average drops from 8,77 to 8,06 - so NOT below 8. That's a huge breakpoint that 5% fails to meet. Thus, there might not be much synergy there. 6% would mean 1.26 attacks per turn, and most importantly, this hits the breakpoint of 7.93 turns per burst.
Meanwhile, 10% elemental attack (from double Reiki) is... in best case scenario ~6% increase. In worst case scenario ~4% (elemental advantage + elemental Relic weapon).
Overall, I'd say that Tiamat is about as good for damage as Reiki is. With Tiamats, your bursts will be slightly weaker, but you might have them slightly more often. And your skills will hit for a bit less.