Naive question on casting 500+ fatigue spells

[tinkthank]

Originally posted by Graeme Dice:
For a better explanation, Solar Brilliance is an astral 5 spell that requires 5 astral pearls to cast (and hence requires 500 fatigue to cast). I have personally seen astral 4, and astral 5 mages cast this spell against a horde of undead without going over 200 fatigue. This is only possible if they are capable of spending the 5 gems as the base cost, then up to 4 or 5 gems more to boost their power level and reduce fatigue.

ALthough couldnt it have been the case that Banner of the Northern Lights, Power of the Spheres, and other boosting spells/items were in play here?