According to a strict reading of the spell, I would have to say no. Shadow evocation is a spell of the Illusion school and the Shadow subschool. The spell description does *not* say that when casting a spell of a given elemental subtype (force, fire, etc) that the shadow evocation is considered to be a spell of that subtype. The gate spell, in contrast, clearly describes when the spell does and does not gain additional subtypes.
In practice, though, I would have to agree with Lord Elias. I would rule that it is a full force effect, unless the target disbelieves in which case 20% of it is still a "real" force effect.