Contagion+Bane solidifies usage of Garuda in AoE situations. I'm not trying to contest that, Garuda wants that for AoE.
However for single target, granted if the damage is a significant jump, it encourages Ifrit usage and summoners having to think about placement. I'd throw him on Caduceus on the side, ADS possibly (would be harder thanks to the Boss central AoE's but possibly still manageable, granted debate with Garuda constant ranged and Ifrit semi-often DPS). Turn 4 is AoE so Garuda only and I haven't been to Twintania yet but I imagine I'd go Garuda for that.
So actually looking back, you're right there are still balance issues. Ifrit also is going to need a change a radiant shield change to something (perhaps damage deduction when not main target?), but while my idea didn't solve the debate, it still makes Ifrit more viable in situations then before.