I can't see using both preparation time and a reagent system to balance the more powerful spells, and given the amount of things left to do, don't think switching to a reagent system is likely if level requirements, preparation time, adjustments to stats / skill points and the spells themselves, etc, will work.
Personally I'm not a fan of reagent systems, or food requirements, etc, so if Larian decided something like that was required, it would be much faster and easier (as stated above somewhere) to just eliminate the spell books and have the highest level spells only available from scrolls.
