I guess it would be possible to be too stiff. However, with that being said, you stated yo have a center shot bow (almost all are these days) and you use a fall away rest. Therefore You don't have to worry about a shaft being too stiff. There is nothing contacting the shaft upon release.
Needed flex comes into play where the arrow needs to bend around the bow when shooting a recurve or longbow or over the rest when the rest is fixed. Note when shooting with fingers the shaft will flex left to right but when shooting with a release the shaft flexes up and down.
So basically what I am saying, with your set up, yo can shoot a really heavily spined arrow and not worry. The increased weight will give you better penetration. My set up is similar to yours and I always use and arrow spined one step up from recommended.