For any of you out there attempting to convert XPS to PDF using an open source means, I was able to successfully do so using PDFSharp version 1.31. It successfully upgraded to .NET 4.5 and I included the PdfSharp.Xps project into my solution. Once you include all the references, you only need a single line to link to your XPS file.
XpsConverter.Convert("D:\\Example\\test.xps");
Note: There are also other constructors available but I used the most simplistic one in my example.
Hope this helps!