Could you try cropping out the empty white space in the left and right areas of your header graphic and then add this custom CSS instead of what you have now:
header hgroup {
background-size: contain;
}
.custom-header {
background-position: center;
}
@media (max-width: 639px) {
.site-header {
min-height: 163px;
}
header hgroup {
background-size: contain;
height: 62px;
}
}
@media (max-width: 640px) {
.site-header {
min-height: 163px;
}
header hgroup {
background-size: cover;
height: 76px;
}
}