idk if its bases on Enhancing Magic but im a 42 rdm and generally it only does about 9 damage if the mob is weak vs fire. i have seen a 75 rdm doing 18 with enfire so i think it increases with lvls.
I'm 66 rdm, and en spells are based on enhancing, when I was like lvl 54 I was doing about 10 additional damage, a few more enhancing lvls later (still lvl 54) I started doing 11 additinal damage. now at lvl 66 I'm doing about 15-16 additinal damage