I guess we have a different idea on what is best for each boss fight and a different line of thinking. To me, being able to do the content in that gear isn't enough to make it the best gear set for that fight. Your line of thinking (correct me if I'm wrong) is that any sacrifice of survivability for damage is worth it as long as you are still able to survive the encounter. If you're able to pump out more damage and have enough HP to survive the encounter, it is therefore better. My line of thinking is more along the lines of which gear set will give us the highest percentage of success and the lowest percentage of failure. What is the single thing that is most likely to go wrong in the fight? If that thing happens, which gear set will give me the absolute best chance of fixing the problem for my party?
If I did 10000 attempts in i90 VIT, 10000 attempts in i90 STR, and 10000 attempts in i70 pentameld. Which would yield the highest win percentage?
I do a ton of EX Primals. My FC sells EX Primal carries (7-man) for gil. I help out my less-experienced friends with their weekly clears. I join PF groups to help out friends. These are all sub-optimal conditions, as opposed to 8 fully geared and experienced veterans. I want to completely minimize wipes with customers. I want to give myself the best possible chance of carrying weaker parties.
I've had situations where the other tank accidentally died in EX Titan and I had to tank 4-5 stacks of Mountain Buster. I'm not sure if I'd be able to do that in i90 STR gear and 30 STR attributes. I've had situations where too much DPS died and we won by the skin of our teeth, and there was no room to compromise my damage output. I feel that i70 pentameld is perfect for this fight. While damage is the bottleneck in this fight, I still think there's a line where VIT into STR sacrifices are no longer worth it. Going from i70 Pentameld to i90 STR is a loss of 50 VIT and 60 Parry to gain 20 STR, and I don't think it's worth it. You might feel differently about this fight (and many other fights), and that's fine. Neither of us are necessarily right or wrong.
Anyway, to answer your question: Yes, I have i90 STR Accessories. My alt is a full i90 Monk. I've tried it vs Titan but not Ifrit.
My i70 Pentameld vs My i90 Strength Accessories:
-50 Vitality
-60 Parry
-19 Accuracy
-2 Determination
-2 Skill Speed
+20 Strength
+11 Crit