[CUnknown]

I use empowerment as Ulm (usually in either blood or nature) to let smiths use their forge bonus on different kinds of magic items. I wouldn't ever use empowerment with the intention of casting a battlefield spell. I think just giving your mages the gems in their inventory before they go into battle is effective. For every gem they burn, they get +1 path for that casting only.

Edit: I guess if you intend to do this a lot, it may be cost-effective to empower instead of using a gem every time. But it would have to be a lot of times, right? And then you run the risk of that mage dying and wasting your investment, too.