Hm, even given all that, it seems like a pretty big thing if a spell nearly doubles the to-hit/parry chance of a fighter for a whole combat - and if the characters have a few rounds to prepare (like when they are seeing a horde of zombies shambling closer), it is really not that difficult to manage casting one spell...
I don't get the reference about the lowered spell resistance, however - I'll have to read up on this.
Anyway, I never really played any of the systems where items give you huge bonuses (and never really liked the concept ...). So to me, this seems pretty powerful - still, not a gamebreaker, and if I continue disliking this element, it is easy to houserule (making Weapen Enhance 2 spells, for example, one enhancing the skill, the other one the damage, or doubling its cost ...).