At tomshardware.com and because of how the entire system works, can amount to no differences. If HP will not answer this, then you have a sign you should shop elsewhere.
I am about to purchase a machine that will be running CPU and memory intensive programs. I am looking at the HP Z820 workstation with the E5-2680 8 core processor. I have a choice between (16GB of) unbuffered and registered DDR3 ram. What kind of (percentage) performance penalty might I see if I go registered over unbuffered?