It might have been possible for Quirrellmort to win the war in a few days, but I think the implication is supposed to be that he could do so by superior inventiveness and planning ability, rather than by superior dueling power. Indeed, going by his demonstrated methods, he might have been able to defeat Dumbledore in the war in spite of dramatically less dueling power.

Minerva has apparently survived skirmishes with Voldemort where Moody or Dumbledore were at her side on different occasions, and if Voldemort could have overcome his opponents in those encounters with no risk to himself, I think he'd have had a hard time rationalizing passing up the chance.