By Solrain 2013-07-04 12:48:23
Bonnet +2 has to be equipped at the time the spell finishes casting. Mortarboard +2 can be equipped for precast to get the casting time bonus, but has to be kept on when the spell finishes to get the recast bonus.
So precasting in Mortarboard and finishing the spell in Bonnet +2 is the best of both worlds.