Excel sheet can write only up to 256 columns - C# (Office Interop)

Hi, yes in this case you will probably be better off with an OpenXML SDK.
You see MS Excel is an UI application so its Excel Interop dll was designed for that purpose and because of that API is very slow and consumes a memory pretty fast. You can read here more downfalls of using excel interop in C# applications.
Also here is solution that you can try:

var data = new DataSet();
data.ReadXml("file.xml");

var file = new ExcelFile();
foreach (DataTable table in data.Tables)
{
    var sheet = file.Worksheets.Add(table.TableName);
    sheet.InsertDataTable(table);
}
file.Save("file.xls");

I used this excel component for C#.