Pure DEF is only great when reaching a certain number and beyond. If you can't reach 80+ DEF, I'd say don't even bother having a Garm.
It's a specialty route in my opinion, so I feel you would be better off just relying on Reductions instead. Example for Nif; Bathory/Evil Druid, Tirfing, Deviling.
Obviously having those in some nice +7's or greater will be an added bonus. Since alteast average DEF combined with full Reductions will be better then just full Reductions in +4 or lower crap for lousy DEF.