there was a massive topic about the Necromage perk and all the effects that it buffs as a vampire and what does / doesnt work etc.
i would say they definatly last longer and yes they probably would be more powerfull but if im not mistaken the master level spells in this are of Daedric quality anyway? so more than sufficent.
sadly i cannot find the topic: ''am i the first one to discover.....'' it was titles something along those lines. if i find it i will link it for more difinitve answers for you.
and would you beleive it! : http://www.gamesas.com/topic/1332899-am-i-the-first-to-discover-this-synergy-with-necromage-perk-and-vampirism/page__st__90__p__20114661__hl__necromage__fromsearch__1#entry20114661
found it right after i said i couldnt >< im sure your answer will be in there somewhere for you, if not thats THE place to ask.