Contagion or not, I'd rather use my pot CD for an X-Potion of Int instead of a poison potion but that's just me...

About Garuda vs. Ifrit
Even without Contagion, I still prefer her over Ifrit just because her style of her AoE. Ifrit's is really narrow and initiates around himself while Garuda's goes off around the selected target - helps a lot in large pulls.
Shadow Flare + Miasma II + Garuda AoE + Summoner's attacks can dish out some pretty decent AoE damage. (+ Enkindle of, WHEN it's up that is >_> , dat 5min cooldown)

Also - Any idea where Ifrit's Radiant Shield is useful?

Still, I'd say the true mastery of this class comes with a lot of practice and dealing with all kinds of people and classes/jobs. You'll have to adjust yourself to every situation accordingly and communicate with others to get the most out of your dps (like telling others to lure adds to the boss for bane or move close to Titan when they are about to get jailed).
You're might be able to pull out super duper dps if you do brayflox HM speedruns all day but I'd take the more fun, unpredictable situations any day - there's where I truly shine as SMN.