This has already been discussed.

Contagion is like a 3% DPS increase, just like RS is only a 3% dps increase.

Its really not as high of a margin as you think.

The convenience of contagion is nice, of course, but if ifrit did more damage I wouldn't hesistate so use him.

Contagion gives you roughly 8 GCDs every 3 minutes (since every 3 minute snapshot has you using contagion 3 times).

If we assume 2 of those GCDs are during RS, then you get 80*(1.2*2+6) = 672 potency worth.

This is equivalent to 3.73 pps, which will be a ~3% dps increase.

In order for ifrit to match that damage, he needs to do 11.2 more pps than garuda every 3 seconds. Even ignoring autoattacks, if they just fix the damage on Ifrit's attack, and its bugged cooldown, he would give garuda a run for its money.