* deimos:

The GPUs are 100% identical, that's a fact. The difference lies in the
functionality that is activated by the driver.

Do you have a source on this?

Yes, but none that is publically available.

I used to think the same, but I've seen
anecdotal evidence that the cores have different transistor counts and
certainly there are different surface components (accounting for the
12-bit color output precision and other media production centric features).

These "evidences" are, as you said already, anecdotal.

Many professional applications like the ones I work with actually have
some code for Quadro specific detection that goes beyond just the driver
or device ID,

Which applications are that and how exactly are they trying to detect a
Quadro beyond driver information or device ID?

so there must be an API method of detecting Quadro
features other than just DXCaps or GL extensions.

Nope, there isn't.

Benjamin