Great work on the new updates on the ZEN theme. I never heard of SASS before I installed this new version and am enthousiastic. Nevertheless, I run into an issues in the fixed-width setup. I think there is a bug in the SASS file fixed-width.scss.
I made an installation with a sub-theme of ZEN. I use fixed width and only 1 sidebar (right) and included a block to make the sidebar visible. I made the appropriate adjustments in the stylesheet list in theme-settings.php. I deleted all -rtl files and the responsive sidebars.css. I did not make further adjustments except changing the background-colors of #navigation, #header and .sidebar.
The navigation bar now seems to be wider then the template. The excess width is equal to the padding and seems so be solved if the width of #navigation is changed into (Line 86 fixed-width.scss):
width: $zen-grid-width - $zen-gutter-width;