my understanding and personal observations are killing mobs is up to 3-4 levels difference has an effect. beyond that the effect is negligible. That is if I'm 4 levels above the NPC i get almost no XP (though still can level weapons/skills a tiny amount oddly) the closer to my level the better the XP and above my level i get additional XP. but as far as i know 4 levels above me or 10 levels above me is no difference in XP, just the time it takes to kill if i can kill.

optimum killing/farming is my level or 1 level above for me in terms of time/difficulty/repair bills. So yes you can farm V16 mobs, and it is nice XP in IC, craglorn Xp is still pretty duff. But you're probably better farming closer to your level. 3-5VR in cadwells silver and gold locations till you are higher level.

Edit:
If i were you id do cadwells until V8, then I'd try going to cyrodill to kill enemies in delves.V8-14 then hit IC. if you have a group for either activity you can probably get by at lower levels. though i see all levels in IC so get a group and try it out if you dont want to wait.