Hi Josh. When I checked your site it appeared that the slider was being cut off on the right side instead of covering the sidebars as you mentioned above. Maybe that's due to the changes you've made? Anyway, I'm not sure it's a z-index problem. It looks like the slider doesn't scale the width correctly. I added this css and it appeared to fix the problem:
.bx-wrapper {
max-width: 99% !important;
}
You need the !important property because the slider container uses inline styles. You might give that a try and see what you get.