The reason I don't usually nuke in +MP 'til it's gone (then switch to full INT) ... is I only really play BLM solo, and I usually open with an enfeeble.
Maybe I'm paranoid in spite of full feeb merits, but on VT's and stuff I still use a very heavy +feeb/+acc build, ~290ish, that brings my MP down to roughly full-nuking level anyway.
Ironically this is the only time I ever use my FMP build. :P Solo stuff is more likely to not be resisted (thinking puddings and wamoura) than a NM or something tougher.
Usually if you are fighting something (solo) that doesn't detect by casting, it's better to pop a nuke on it first and then follow through with your enfeeble. You can even cast enfeebles in FMP gear if you have merits and such. Even if it does detect by casting, you can get just outside its cast range but just inside your targeting range to nuke it without it detecting (if it doesn't move while you are casting).
Also, it's handy to keep elemental seal available for when you are nuking in FMP gear where you are more likely to get resisted (at least on puddings).
1) I have around 1280 MP in FMP gear as a mithra BLM with MP merits. My nuking set has about 890 MP in it when casting. My enfeeble set drops me down to 720 MP.
2) While I am very reluctant to even compare Ixion to Gleeman (because, really, Ixion is one **** of a cape worth the price tag), I'd say that the -MP from Gleeman doesn't really matter if you're a taru.
3) If you are nuking in hierarch belt over obi and zenith pants or some other MP pants over AF2 legs on the appropriate day, no it definitely isn't worth the loss in damage. Otherwise, it depends...you really should have a 'semi-FMP' build macro anyways.
Like others said, make a couple macros that gradually switch out your MP gear with your INT/MACC/MATK/etc gear. Or get used to swapping in pieces manually while casting. Also prioritize which pieces should go in first over others. MACC is nice but you most likely aren't going to need it when soloing puddings.
BLM can be pretty complicated if your macros are not set up right.
I suggest the following (this is the macro system I use):
1) Make 3 books (at least) if you have AF2 pants or obis; I highly recommend more (like a book for casual play and a book for endgame mobs where you switch out more MATK for MACC/INT/etc)
2) Label the books ice/thunder/normal/+any other spells that you have the obi for (or AF2 pants, really)
3) Configure all your nukes and enfeebles using this pattern:
/ma "Spell here" <t>
/equip main "Appropriate elemenatl staff here"
/equip waist "Appropriate belt here"
/equip legs "AF2 pants here on ice/thunder/fire/etc day macros...on your 'no day' book, stick in mahatma/morrigan/whatever
4) Make separate macros for FMP build, HMP build, latent build, enfeeble build, -enmity build, INT build, and a set that mixes you in a suitable amount of +elemental skill/MACC/INT/MATK for things where you may be resisted
5) Set up a standard 'nuke' macro but do not include 1) your main weapon (mentioned earlier, your macros should already have the appropriate staff) 2) waist slot
When you're finally ready, it'll look something like this.
1) Hit your thunder IV macro [equips your thunder/jupiter staff, penitent's/witch sash/rairin obi, and AF2 pants/mahatma/morrigan pants]
2) Hit your latent macro [equip for sorc. ring latent...make sure waist slot and leg slots aren't included]
3) Hit your 'nuke' macro [equips every other slot appropriately with your average nuking gear]
Make sure to do something like this when it comes to sorc. ring:
/equip ring2 "Sorcerer's Ring"
//equip ring2 "Omega Ring"
//equip ring2 "Snow Ring"
Sometimes you will not want to be in latent HP to nuke. In those situations, comment out sorc. ring and uncomment one of the others.
This way, you can easily nuke in FMP gear but still have your staff equipped and bonus damage from AF2 legs and obi. I know it looks complicated but it really works wonders for me. Edited, Jun 27th 2009 3:01pm by HitomeOfBismarck