I don't believe that would be heresy at all. Jesus, being fully human and fully God is the only one qualified to act as high priest for the church - the go-between between God and man. He's qualified because he was tempted in every way, but was without sin.
At least this is my understanding, and I tend to be pretty conservative when it comes to my understanding of Scripture. Mainstream Christianity would likely argue that this is the only way to understand this particular viewpoint. If Christ was only God, and was not also man, then he is not qualified to pay the penalty for our sins. He would have no temptation, being God only. However, if he was only man, he would not have had the capability to be sinless, as all mankind are fallen and depraved. It's only through the combination of the two natures of Christ that Christ's sacrifice is both unique and satisfies the requirements of the law.
I'd like to hear a Catholic chime in on this one. From what I understand, some in Catholicism believe Mary to have been sinless, which is not a viewpoint held by mainstream Protestantism.