After messing around with the CSS layout and a full-sized logo, I changed things up a bit, using two logo sizes and this add-on so the logo would fill the complete logo block area. Works perfectly with XF 1.5.11. I only made one change to respollo.css, changing three instances of @maxResponsiveNarrowWidth to @maxResponsiveWideWidth, so the smaller logo would kick in at 800px.