Reactive Counterspell:
You may do a counterspell in place of a defense roll, making your Lore assessment as a free action. Otherwise, treat the counterspell normally including taking mental stress. (YS253) [Notes: This give the caster more options and is particularly against high control, low power practitioners]
This would take away some of the need for wizards to plan ahead rather than react to be strong. I would probably not include this, but that is just personal preference, mechanically I don't see any problem.
Mental Compartmentalization:
You may take an additional minor mental consequence related to spellcasting.
There are other stunts that do the same for other consequences/situations, so this should be good.
Rapid Extension:
By inflicting a 1-stress mental hit on yourself, you may prolong a spell for a single exchange as a supplemental action. (YS259) [Notes: This is trading 1-4 shifts depending on Conviction for the ability to do so as a supplemental action]
I understand this different than sinker, especially since taking 1 mental stress to extend the duration is pretty much standard if you are already at your conviction level, powerwise.
My interpretation: Let's say you put up a shield spell that is about to expire, but you need to shoot at that one guy running while the other three are still shooting at you. You shoot with a supplemental action of extending the duration and the shield lasts another exchange.
This I like, it adds a bit drama to a fight by draining the wizards resources even quicker.
Shaped Effect:
You may ignore a target as part of a zone wide evocation attack. Because you must control the energy around the target, the spell requires an additional shift of power for each target ignored. (YS251) [Notes: The power essentially represents it being more difficult to control and more stressful to cast, rather than actual power]
I can see this create all kinds of cool effects, so I'm all for it. It is going to be pretty impractical, if there are more than one or two friendly targets anyway.
Reactive Block:
You may perform an evocation block in place of a defense roll. Otherwise treat the block normally, including taking mental stress and rolling to control the effect. Optionally, you may instead have the effect work as armor on a failed defense roll, taking mental stress and rolling control as normal. (YS252) [Notes: This is less powerful than an enchanted item, but gives the option. It can also be prolonged, where an enchanted item can't]
It's basically moving a trapping to another skill AND you need to pay mental stress to use it. Feels balanced enough. Though enchanted items might be a better way to go, since you get 4 of them for 1 refresh.
Rapid Thaumaturgy:
Choose a single type of thaumaturgy. You may perform this with evocation’s methods and speed. (YS288) [Notes: I don't like this costing 1 refresh for flavor reasons, but can't find a reason to justify increasing the cost. Not sure if I'd allow it for thematic types]
Look at sponsored magic, places of power. Those cost 3 instead of 4 refresh, because they are limited to a location. Now if you give a sponsored magic a similar limitation, only that it isn't geographical, you would be there. With evo+thaum it would only cost you 1 refresh, just like your stunt. I would prefer the sponsored magic solution here though, giving it a bit more flavour.
Enhanced Item:
You may prolong an enchanted item effect as you would an evocation effect. (YS259) [Notes: This could make evocation blocks useless for some casters, not sure if I like that]
If I look at this correctly, then it should only effect blocks. I would probably let enchanted items do this anyway. That would be 2 uses and 2 actions spent for a prolonged shield or other block, which can be pretty long in a fight. RAW says an enchanted item always does the same thing, but I kind of see "putting up a shield" and "prolonging a shield" as similar enough to let someone do that without a stunt. And prolonging the effect of an enchanted item with evocation should work without it, too.