I was having the same prob with IE blurry header image. I noticed the header image resizing a few px away from original dimensions. I wondered how efficiently IE dealt with the new REM values, so I commented it out for the site class in my child theme. That seemed to fix the problem for me.
.site {
margin: 0 auto;
max-width: 960px;
/*max-width: 68.571428571rem;*/
overflow: hidden;
}