If you change the Boot Mode from UEFI to Legacy, you'll have to reinstall Windows 8. So you may want to image the entire hard drive before doing that, just in case you decide to undo...
Unfortunately, Dell and the other PC makers are stuck with a lot of things being forced on them by Microsoft.
You might search these forums for other threads about installing Linux on the XPS 8500.
If you have Win 8 Pro or Ultimate, you have free downgrade rights to Win 7 Pro, which might be more tolerant of your dual boot arrangement. But you will have to install Win 7 in Legacy mode with Secure Boot disabled in BIOS.
Without Secure Boot enabled in BIOS, you'd be no more at risk than you would be with Win 7 on any older version PC.