Decent Speed Infusion on a F/P Arch Mage - is it worth it?

Nope, staff speed has no effect for adventurer mages.

@Green4Ever
You can always do a extra run of dojo using the +1 casting speed buff in the rest area.
That should get you a pretty good idea if decent SI is worth it or not.

That's what I thought. I just had to shove it all in my friend's face.