You didn't even factor in what the 532 MP is worth in an encounter yet if you will inevitably go OOM and how that 532 MP relates to forcing you to use an Energy Drain to stay afloat over a Fester for pure damage.
Assuming you are not using AF stacks for anything else / or reason and purely going for Festers, it takes you 30 seconds.
How many Ruin II's you use is mostly irrelevant in this case. Just because you got MP in the beginning to counter balance the SMN switch, that does not take away from the fact you lost MP to use later on.
What is the difference If I didn't make the switch? I just have an extra 532 MP to use now towards general casting and DoT upkeep. That's why I say, in a fight you will inevitably hit OOM when maximizing your rotation for DPS (all of FCOB), that 532 MP becomes way more valuable to keeping your DPS up.