It seems like Legends and Lore priests should use the specialty priest XP charts from Faiths and Avatars, curtailing this sort of build.
Well, to be honest, I don't think ANY specialty priests should use the XP chart from Faiths and Avatars. Basing it on the Druid charts was just foolish; there's a REASON Druids have the bizarre fast-then-glacially-slow-then-fast-again-then-slow-again XP progression, and it doesn't apply to other specialty priests. I'm not getting the Grand Druid's 6-spells-per-level, I-rule-the-whole-world goodness, so why in the world should it cost me 1,500,000 experience points to go from 14th to 15th level?
That said, my case for NOT using those rules is as follows:
1. Faiths and Avatars is a Forgotten Realms specific supplement.
2. Faiths and Avatars does NOT say that those progressions apply universally to specialty priests in all settings.
3. It's not reasonable to publish rules in a setting-specific supplement (and nowhere else) that are meant to apply universally to all settings.
4. Legends and Lore makes no mention of special experience tables for the priests described there. Nor do any of the other sources I'm employing in crafting my character.
5. Legends and Lore came out LONG before Faiths and Avatars; clearly, Legends and Lore was NOT designed with those rules in mind.
6. Moreover, a case that the F&A rules DO apply would require a massive retconning of existing characters at the time of release. (Should a level 20 L&L cleric suddenly drop to level 14?)
7. The priests of Math Mathonwy are specifically described as "clerics or druids"--thus, by the letter of the rules, they should use the cleric or druid table, depending on which they are. Mine's a cleric.
Have I missed anything? I think it's a fairly strong defense.
Yes, if the DMs decree that the F&A chart applies to all specialty priests, I'm hosed and will need to go back to the drawing board. I've sent them my character rough draft, so hopefully they'll get back to me.
That said, Math Mathonwy (urge to dig out my Mabinogion rising) is by far the best choice. The only other ones that at all compare are Isis (multi cleric/wizard, auto passing all magic saves), O-Kuni-Nushi (cast from one wizard school, due to lack of phrasing 'as priest spells of same level' can be interpreted as seperate progression), Aegir (fighter that happens to cast as a priest from the sphere of weather), and Fu Hsing (casts as bard in addition to cleric), as well as a couple of the ones that get high hide in shadows/move silently.
The problem with Fu Hsing is the SEVERELY limited spheres. The bard spells are nice, and I love the "Cast any spell by playing a single note on a musical instrument" thing...but he just lacks raw spellcasting "oomph." I thought about Isis for the reasons you mention (and the magic resistance, which my current build lacks,) but anything that doesn't hit ninth level spells just isn't going to pass muster, I think.
I'd add Horus and Anhur to your list. (Priests of Horus are paladins who cast as priests from the spheres of all, combat, and sun, with minor access to guardian, healing, necromantic, and protection; priests of Anhur have the same spheres, plus many of the goodies of a fighter.)