Do I lose the increased audio fidelity benefits of the Core3D processor in my Sound Blaster Z when I use a USB wireless headset like the Logitech G930?
Most definately, it's not just a totally different audio device you would be using, but the good DAC and amplification the Sound Blaster Z has can't be used at all, since the D/A conversion has to take place in the headset itsself, which usually has vastly inferior D/A conversion than a good soundcard.
long story short: If you really care about audio you basically can't use any wireless headphones
I use the HD598's which are a good pair of all-rounders that are known for their soundstaging for gaming. If you want a more comprehensive list, check out this thread: