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Graeme,
I'm not convinced by your arguments :
Nature *has* some good spells, sure, but that does not make green gems more valuable than the others. IMHO they are often pretty much useless unless you play a nature-oriented nation, which is not the case with Astral, Earth, Death or Fire gems. The only less useful gems are Water...
As for Vine Ogres being inferior to Lamias, true again, but Ogres are a lesser summon and rather make better damage-absorbers than Lamias. Lamias require a higher level to cast effectively, Vine stuff having easy "improving" items.

Eventually I don't think Ogres need a boost - or rather all other low level summons require a much more serious boost !