I simply don't think inside of Abyssea for balance adjustments. Frankly, I don't know why anyone does. Abyssea isn't balanced at all, and hardly any job needs any real adjustment to be viable inside Abyssea (you could throw practically anyone in there and they can half-ass their way to success with the right atma and temp items). I feel it's everything before Abyssea that matters in terms of balance, and of course.. eventually what comes after Abyssea, assuming much of anything ever does come after Abyssea.
I'm not saying it wouldn't be nice to do this, it would. It'd be handy and it would give SMN something to actually do for a change in the endgame. But I'd rather have something that was more globally useful than something I can only make real use of in Abyssea.
I've honestly never noticed the spirits having defense relative to Elementals. Admittedly I've never used them in a serious encounter before (for obvious reasons) but at the very least.. NPC spirits certainly die a hell of a lot faster than elementals. So unless this is just a player exclusive thing and I just never noticed because they're too unreliable to pull out into a serious encounter.. I don't know. Even with high defense though, I don't think they have considerably high health. From what little I have used them, I seem to recall them dying pretty quickly actually.. but it has probably been years now, so who knows.
And they don't necessarily have to remove perp or make it significantly lower, that was just a thought given how meh they are right now. I do not think they should cost more than the avatars though, and I'm iffy on them costing the same as avatars too.. Maybe a couple of MP less? Something around there anyway, it's silly for them to be so expensive.