Thanks for the response.

I think it was just a coincidence that the eGPU started working with acpi=off. 
Taking a closer look at the issue it really does appear to be a BIOS problem
that prevents the proper PCI resource allocation to one of the TB PCI bridges. 
In fact when I took a closer look at the dmesg with acpi=off I still see the
resource issues present.

I've opened up an official bug report here with the kernel ACPI BIOS team here:
https://bugzilla.kernel.org/show_bug.cgi?id=201527

I realize the issue should really be solved by the manufacturer, but perhaps
the kernel devs can create a work around and/or have more direct lines of
communication with the Dell engineers.  Thank you both for your suggestions and
comments.

Rob