You don't need to worry about MP upgrades. By the time that you've farmed a S class soul, you'll have more MP than what you can possibly use. I'm currently floating 900+ MP which I cannot use.
Remember that if you want to change MP upgrades, you can always remove points for free. You don't get the MP back, but again, that shouldn't be an issue at the point where you move to an S Class Soul.I may have glossed over the fact that Parac will allow your team to generate Burst, which makes her goddamn good for any content that lasts for over 10 turns. Whoops?
But yes, Medea will still be the farming queen, and Yori ought to still be the raid farming queen.