I always hated that too. If you wanted my suggestions, I would have the helm give constant, low level of power just from having a spellcaster sitting in it - but if need be, the caster can expend spells to increase speed and/or maneuverability, the amount gained based on the spell expended. Some helms might have a greater rating for how much you can 'overcharge' them through this method then others, making it valuable to have both a powerful spellcaster and a powerful helm.
J