Just a few days ago, Anand stated quite clearly in an interview for Spanish newspaper El País, that no top player should have any trouble beating Fischer or Spassky. He added it would be much different if he was given 3-4 months to get updated.
Reference: http://www.cadenaser.com/sociedad/audios/viswanathan-anand-alguna-forma-cerebro-funciona-forma-distinta/csrcsrpor/20140921csrcsrsoc_11/Aes/
NB: (around minute 11) Audio is in Spanish.
Bobby Fischer was genius, even without new knowledge, he would still beat GM's <2700