I could of SWORN the Imp call spell does more damage to other Imps. Test it again, I'm 99.98% positive it does.
I tested against all the Imp enemies. There is no bonus.
If you plugged in the numbers to the magic formula, the damage range when Rydia's Wisdom is at 99 (meaning 25x spell mutliplier) is 200-300.
Every Imp varient,
except for the Imp Captain takes full damage.. but that's because they have
0 magic defense.
The Imp Captain has a magic defense of 7, which limits the damage output significantly.
The other enemies around Baron,
excluding the SwordRat have 0 magic defense.
In fact, the SwordRat has enough magic defense (11) to fend off the Imp summon, which means the damage dealt is always 1.
There is no multiplier, and feel free to test that yourself.