Fire/Poison Mage Builds Post-Big Bang?

I'm a bit puzzled by this myself. I think I'm okay up until 4th job, with this as my planned build:

1st Job
Max MP Boost
Max MG Guard
Max Magic Claw

2nd Job
Max Meditation
Max Teleport
Max Fire Arrow
Max Spell Mastery
10 Slow

3rd Job

Max Element Amp
Max Explosion
Max Poison Mist
Max Seal
Max Spell Booster
Max Teleport Mastery


For 4th Job... I'm not sure. I'm currently level 150. I was planning to max Paralyze as standard, maybe get MW20, then after seeing how good Big Bang is substituting it for Meteor Shower. The one thing I'm concerned with the Big Bang skill is that from level 1 - 30 you'll only get a 57% damage increase, and everything else is the same apart from a higher MP cost. After trying out my mage yesterday without touching my skills (i.e. no spell mastery etc.) I could hit 150k on Fire weak with Big Bang but that range was quite wide, all the way down to 30-50k. That's at level 1 Big Bang. Is it really worth spending 29 points on a 57% damage increase? It looks kind of shabby to me...

Was also looking at maxing Ifrit. Meteor Shower seems kind of useless to me now, hence considering my build without it.