I am sorry mate, but I've tested this on my own rig and and i get anywhere from 2-5 additional fps with the addition of the sound card. (the variation is game dependent)
I don't understand why you don't but It doesn't really matter.
I do not understand why you have that much RAM. I can only assume it is overkill if you don't do any virtualization or use graphics rendering software.
But this is off topics. and the point is that I do see an increase in performance in my current rig.
However, I would like to hear your argument about why USB is the best interface.
Maybe the newer sound cars have better hardware or software for off loading sound processing. I did have an older sound card. I actually pay around with ram disks that's why I have that much and because it was cheap. In any event what I said about $200 invested in other PC sub systems netting more FPS per $ spent is still true.
USB is the best because its plug and play, if you use a sound card or MOBO sound chip the soft ware and or drivers have the potential to become unstable over time and crash. Also glass optical cables can break if over bent and both glass and plastic can get starched easy. USB also has lowest potential for forms of digital distortion and less susceptible to all the electrical noise that is in your PC. Aka less likely to hear audible hiss or static and will get clearest cleanest sound.