No fenrir wouldn't stack. You would take the higher %, in this case the fenrir.
Well now this needs to be tested.
Think about it, this is a set with a base fixed casting ranging from 15 to 25%. It then adds an additional 1% per upgrade level of the suit. So theoretically the additional fixed cast reduction could stack with fenrir, or it maybe a seperate call again which means it does stack.
It would be worth looking into because instant cast meteor would be awesome.