Wizards aren't exactly weak but this might be an interesting way to give them a little something without being TOO devastating.
Wizards can pick one spell to "Master" once they reach level 8-9, they must have school specialization and GSF related to the given spell.
When casting that spell they get +2 DC/CL (maybe just one they pick when taking the mastery) as long as it is benefitting from at least one metamagic feat (silent, still, extend, empower and so on).
So for example a GSF necromany wizard with school specialization necromancy could take ghoul touch as their chosen specialty and as long as they cast it with something like extend, they could raise the DC by 2.
This sounds pretty devestating.
I think this is a good idea as long as they are only allowed to choose from the spell circles 1 and 2.
A lvl 9 wizard with GSF, casting an extended combustion or Tasha´s, do not need to be made even more lethal.