The only time -res gear on you work elsewhere is, afaik, are sin traps and hydras which are special cases. Outside of that its a big fat "NO". You're a necro, be satisfied with your amp and decrep already.
I mean if I use Guillaume’s with -10 jewel instead of Cham the he will be at 65% phys res against immunes rather than 75? 95 - (100/5(20))