AFAIK impedance mismatch should only matter in relation to speakers and amps or headphones and dacs/pre-amps. It shouldn't matter when doing line level output to an amp or even a line level loopback (dac to adc). The fact it has such behavior in a consistent manner leads me to believe it is just out of spec or badly implemented, even if it is impedance matching after all.

Regarding noise floor, the "DAC" (really, the audio source as a whole) did matter in this case. The volume is being set at the amp to as loud as I need because the volume is being controlled at the source. In the xfi setup I couldn't hear any hiss, noise or EMI from the pc itself, while on the onboard audio it bothered me pretty much instantly with a constant hiss and EMI buzzing even from moving the mouse. I do want to note again that it's likely the manufacturer implementation at fault rather than purely the DAC fault, regardless, the user is the one being harmed with a subpar product.

I do my setup this way because I have a pile of DSPs running in that PC, including an equal loudness contour compensated volume "dial" (bass/treble gets boosted as volume goes down essentially), so controlling the volume at the source is a must.