The element you want to resize is now called the .mx-datagrid-content-wrapper, and tundra isn't necessary either.
You should be able to use the css class below:
.wideGrid2000 .mx-datagrid-content-wrapper {
width: 2000px;
}
What Jasper explained is true, but to achieve an independent horizontal scroll-bar you need to:
.horizontalgrid .mx-datagrid-head-table {
background-color: #ffffff;
margin-bottom: 0;
position: relative;
width: 1500px !important;
}
.horizontalgrid.mx-datagrid-body-table { border-top: medium none; margin-bottom: 0; table-layout: auto; width: 1500px !important; }
P.S. I've added "!important" because I'm lazy, you can choose to set the update the class a level up too